Including on a bike, whether it be a long distance tour such as crossing a continent, a race against a pro, or on a pro team. Or a mountain climb, snow skiing, hang gliding, dog sledding, badminton, checkers, Go Fish...
Probably my first best accomplishment was at age 11 and being the only one of 5 (including my brother and cousins) to get up on water ski's on the first attempt. See, in our family, water skiing was a rite of passage. My Dad and his brother's were HS and college sports stars, and all were excellent skiers, so we kids had a lot of pressure on us to do well. Leading up to that day I had read an article in Boys Life about how to water ski. I practiced the seating and step-by-step positioning that was recommended in the article. On the way to the lake that morning I told everyone I would get up first try. "Oh really! And what makes you say that?" "I read about it in a magazine." They all laughed. Being the youngest of the group, I was vindicated by doing just what I said I would do after watching all four before me struggle and fall on their face and asses. Had I known about head nods and gangsta hand symbols, I would have been nodding and flashing at them behind the boat that morning. "Yo fools, look at your boyeee!"
Another would have been my first single day century+ ride at age 15 on a heavy Schwinn touring bike loaded with 20+ lbs of camping gear. The last 30 miles felt epic. Wracked with soreness and exhaustion, there was no question of reaching our lakeside destination. It was either that or death! We finally reached the Great Lake shore as the sun was setting. We sat in the sand wordlessly for a long time before washing up and finding some food within walking distance, then plopped down in the sand and fell asleep like dead dogs. Within a few hours of waking up the next morning we were back on the road for another 50miles to where one of the rider's older sister was camping along a lake with friends. Can you say crazy teen beer party?
My Go Fish story just happened this past Thanksgiving when I taught the 7 yr old daughter of friends how to "properly" play that simple card game. She beat me 8 out of 10 times. Not a personal best, but a proud moment of watching the birth of a true competitor.