Sometime in the late 50's, I got in my mother's '53 Buick Special and knocked it out of gear causing it to roll out of the driveway, across the street and into the front porch of the neighbors house. Luckily all at pretty slow speed and neither the house or the Buick were badly damaged.
When I was 13 my dad was taking me for a ride in his new 2001 Bullitt mustang. He pulled in a parking lot and asked if I wanted to try, and boy did I.
He explained how the clutch and everything worked. He failed to mention the clutch gets stiffer as you take your foot off, however. I started releasing the clutch, and it felt like it kicked my foot off, leaving 2 patches of rubber on the pavement. After some more failure at taking off and shifting to 2nd smoothly, I stopped and had to wait for my dad to stop laughing before he drove us home. He thought it was hilarious that my first driving experience was laying rubber unintentionally in a mustang.
About two and half years old. Living in what might be the only hill in Iowa, I got the Morris Minor wagon rolling down the street.
My older sister, who's fault this all was, bailed out the window half way down and let me take the heat.
I seriously messed up a nice ladies garden at the end of the road.
Must have had an impact, since I have had Brit cars since the day I could drive.
I was 14 and my stepfather told me to get out of the truck to open a gate on the farm (this was in the mid eighties and our farm truck was a '64 Ford column shift). When I came back to the truck he was in the passenger seat and told me it was time to learn.
I remember how uncomfortable he looked because he was very tall and at the time I was only about 5' so to reach I scooted the seat WAY forward.
My first try I stalled it then on my second I sprayed gravel for 50 feet 
From then on I was hooked on internal combustion!
I am surprised to not see this answer before now:
Then moved on through a Honda 100, 5Hp B&S powered off-road cart, Honda ATC70, Dad's Can-Am 175, then eventually to a Suzuki GS400.
I'd been on tractors, mowers and dirt bikes since young. Had a g-kart at 10. But on the street? 13. 1977 Dodge tradesman window van with steering as precises as chinese emissions standards. At 14 dad was teaching me the finer points of drifting the ass around dirt corners.
I've no idea how old I was, 7 maybe? Dad put me in Tommy's go-kart and told me to go down the street and come back up to the house, he was watching from the end of the driveway. I go down the street and get to the cul-de-sac not knowing that half of it is flooded. And not with water, with mud. I drove straight into this giant mud puddle, it soaked me up to the waist and the kart stalled. I walked back up the street, crying and telling and saying I broke the kart. Dad thought it was hilarious. Me? Not so much.
My first time behind the wheel I was 12. I had consistently harassed my parents since I was old enough to see over the dashboard and my mother finally gave in. She stopped the car on a hill and told me, "I'll let you drive the rest of the way if you don't stall it." I was given no further instructions, but I had been watching mom's footwork for years and I knew that rolling backwards was going to be my biggest obstacle. I moved over to the driver's seat and eased in the clutch and selected first. I gave the car a little gas and released the clutch and the handbrake at the same time. There was a little wheelspin, but overall a pretty smooth start. Mom was amused that I "cheated" and used the handbrake, but I'd seen her do it one other time trying to start on an icy hill. She kept up her end of the bargain and always let me drive once we reached the unpaved roads that led to my Grandmother's house. Lots of good memories in that Celica sitting in my garage!
Growing up, both of my folks worked at an amusement park on Nantasket Beach Mass.
http://en.wikipedia.org/wiki/Paragon_Park
They had a ride called the Indy 500, which was a miniature board track. (banked wooden oval) Although I was technically too small to ride-- the operator knew my folks, so he used to let me on anyways. (hey it was the 70s, folks weren't so uptight) My brother and I dominated on that track--- and I'd usually beat him, as I was lighter! 
That track-- and the glorious feeling of victory--- had a lot to do with me becoming a car nut.
On the street-- my first time driving was in driver's ed, in a 1986 Pontiac Grand Am--- during a driving rainstorm. The streets were flooded, I could hardly see, and the instructor kept telling me I could go faster. I've been obeying him ever since!
Model A Roadster, don't know the year, we never had it registered. My dad stripped it out except for a small single seat and we used it to haul stuff around the farm. I think he sold the trunk lid & rumble seat kit to get them out of the way. You could hold a lot more in the trunk with them gone.
The A was kinda handy with the manual throttle and enough torque, no way you could stall it. My brother and I used to get out on the running boards and steer through the window. That way you could haul more stuff inside, yet think you were on a thrill ride. Till we put it so deep into the creek, we had to use the horse team to pull it out before Dad came home!
Yes I said horse team. In our part of the country most hills were too steep for the tricycle gear tractors. The richer families had mule teams. We couldn't afford a mule team but our horse team worked out pretty well. Then the Ford 8N & the Ferguson, don't remember the model tractors, became popular. The horses ended up being retired except for pulling the hay or tobacco wagons.
I never really took to the tractors. I thought the Model A was almost as useful, except for plowing which I hated anyway, and a whole lot more fun.
Power Wheels Wrangler, I was probably about 3 years old.
There was one thing that did work well with a tractor. In nice weather my neighbor and I sometimes dawdled around the house till we missed the school bus. My Dad was at work and we hadn't another car except the Model A. So mom would let us take the tractor to school.
A ten mile ride on a tractor doesn't much like fun now. But at age thirteen or so it was a hoot! Plus we knew if either of our dads heard about it, we'd be whipped within an inch of our life. We were nervous and excited all day long. I bet our teachers were less than thrilled!
Then one day a rich kid showed up on his new John Deere. After all the hubbub that beauty caused, our little Ford 8N never made the trip again. I guess you'd call it tractor envy.
When I was about 7 with my dad and granddad loading hay bales on a wagon. After a few trips around the field, they wanted to get more done, so they plunked me on the tractor seat (ford 9n)and said just do what they had done. A few months later they turned me loose with a farmall cub showing me how to work the clutch and brakes. About a month after that a honda CT70 showed up on the farm. On the road it was my granddad's 1967 ford F100. Straight 6, 3 on the tree, rubber mats... In my mind pick up trucks were always implements. Still hard to wrap my head around the ones that are too fancy to actually do any work - granddad's often had hay bales, tractor parts or a cow in it.
The ford never looked that good, even new - wheels weren't body colored and had ford hubcaps - no trim rings. The tractor had a good layer of old grease and dirt around all the moving bits.
May not have been the actual first time I operated a motor vehicle, but the first time I drove a full sized vehicle was my dad's 76 CJ5. Three speed, HEAVY clutch, bus sized steering wheel and no PS. Fortunately, it also had a bus sized bumper on it, because I missed a corner and hit a tree. No damage. They don't make 'em like that anymore.
Late 1960's mini bike with a B&S lawnmower engine, until my brother drove it into a ditch and broke the steering pivot.
First real driving experience was in Driver's Ed in school, in a Chevy Vega. I remember two events in that car. The first was learning to start on a hill (it was a stick of course), which I figured out okay. The real fun was the girl in our group who just kept pushing the gas further and further down as we rolled further and further backwards down the hill. I never knew that a Vega could make so much tire smoke by dumping the clutch.
The other time we were driving on the freeway and came to one of those huge merges, where two freeways came together, essentially becoming an 8-lane freeway. We were in the slow lane, with the "point" to our right gradually getting narrower and narrower - and right then the engine died. So I'm coasting along and realized that if I let it coast to a stop, we'll be in lane 4 of an 8-lane freeway, with people driving right up our butts. So I stood on the brakes, no doubt freaking everyone out in the car as for why I was doing that, and stopped on the end of that "point." Never forgot that, that sometimes you have to make a decision right now to keep things from getting much worse. I recently contacted my old driving instructor (30 years later) and he still remembered that day... which could have turned out a lot different.
In reply to Curmudgeon:
I remember the King Midget! Never saw one, but my best childhood friend was a true car nut who knew all about them. Showed me pictures and everything.
I think my first driving was with a family friend who had a Beetle. This was about 1960. I couldn't grasp that the car needed throttle as the clutch came out and he was too shy to tell me to gas it so I stalled it about 50 times before I figured it out by myself.
Ford 9N when I was about 10. We were on a camping trip and the RV park owner came out and saw me staring at it. He showed me what to do, then stood behind the seat while I drove it around. Waved at Mom and Dad as we rolled by. This was in the early '70's. After that there were frequent trips to a Kart track. I learned to drive a car in a parking lot. '68 Charger 
Took my driving test in my '65 Barracuda (which I almost failed...Something about breaking traction ) The first stick I ever drove was a '63 Studebaker Lark.
Been riding motorbikes since I was 6. Quickly moved up to quads as well.
Was allowed to steer the car sitting on my father's lap pulling down our street. That really doesn't count as "driving" though.
First time actually driving, I was 14. My dad and I went out for a drive in our 79 Corvette. Around midnight at our return, he pulled into the Meijer's parking lot. It was nearly empty at this time. He let me drive around the lamp posts, laying out a pseudo Auto-X course of sorts. I began to circumnavigate the course, tried to go a MPH or 2 faster each time.
The car is slow as hell, but it was definitely an experience I'll never forget. Probably the reason I lusted after a Corvette of my own for so long.
In reply to Contradiction:
Besides go carts and mini bikes?
My brother car-pooled to work and often left his manual shift Cavalier home. I was 15 with a girlfriend that lived two towns away.
Do the math: I got in, learned REALLY quickly how to drive (and drive a manual) and visited my girlfriend several times per week.
Never got caught... By the law!
Six, my next door neighbor's series 62 1962 Cadillac.
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