my younger son did his first double century when he was 11 and a pipsqueak.
he rode his 10 speed schwinn (what else did they have for little kids then?) all the way to Portland, learning how to draft off large men along the way. Except for occasionally crying when having to ride up a big hill (not on that ride) he was always fearless and strong, as was my older son.
What did that riding at an early age do for them? they are both incredibly confident riders (they know they can do any ride they set out to do) and they both have great legs.