My oldest son Jack, (he's five) learned to ride a bike this weekend. I jogged behind him with my hand on the back of his seat letting go then catching him when he tilted too far in one direction or the other. Finally, I let go and he continued on without me. He was nervous but also very proud of himself. After he "got it", he never looked back. Soon he was using the back petal to break and starting and stopping himself. Two days later, we went for a family bike ride and he lead us all the way around the grade school playground and only came too close the building once.
It's amazing how quickly one learns to ride a bike and even more amazing that one never forgets how to do it. We are very adaptable creatures and it's sometimes easy to see how we have become lords of this planet.