Last year, Timmy and I had decided Nico was old enough to learn to ride his bike without the training wheels. We took them off, tried to help him learn, and after a few tries he insisted on putting them back on.
A few weeks ago Nico decided he no longer wanted the training wheels on his bike. He went to the tool box, picked up tool after tool until he found the right one and took them off himself. A week or so later, he was happily, confidently riding his bike on two wheels.
The moral of this story, kids know when they are ready for something, and it doesn't necessarily correspond to a certain age. Do other kids ride bikes on two wheels at an early age? Sure. Does that mean my kids have to? Nope!
The thing to remember, while unschooling, is that you are letting your kids work on their schedule. Whether it's learning to read, solve complex arithmetic problems, using tools, or riding a bike, they will do it with ease when they are ready. If they are resistant, or hesitant, put the training wheels back on for a while. Let them revisit things at a later date, when they feel more comfortable, and be amazed by how much easier it is for them to learn.