DS has DCD (dyspraxia) and I thought he'd never learn to ride, or he'd ride really late so I haven't tried to teach him. But he was riding a razor scooter over the winter in physical therapy (up and down the hallways of the building). That taught him the balance needed so that as soon as the snow melted off the sidewalks he dragged a bike out of the garage that I had forgotten was there, jumped on, and started riding like it was no big deal, within seconds of getting it out of the garage. I was inside the house and saw him zipping past the house on a bike. It was a startling sight. Reminded me of that scene in the Wizard of Oz where Dorothy is in a tornado and the witch lady rides by the window on a bike. He had never had problems pedaling though and could ride a bike with training wheels since he was 3. It was the balance that he didn't seem to understand until recently.
With DD I had to actually put more effort into teaching her. No DCD but she was younger (like 5)...I had to do the whole thing with running beside the bike, practically holding the bike up, etc. Not so with DS, my uncoordinated one. I think he just matured enough over the winter that he was ready. And the razor scooter helped. Kind of like babies learning to walk. One day they can't do it at all and the next day they can.