XTC1, my daughter, who was always tall for her age could pretty much do it from about 3yrs, while I think my son was 4 before he could fit properly. As soon as they could sit on the saddle at its lowest point and comfortably reach the pedals was good enough for us given that they were already familiar with their own bikes (and helmets) from 2yrs already, albeit that they might have been on training wheels when they started. Like most things, try slowly, ride short distances, working with them to help them gain their confidence and balance. It was great that my wife was riding behind, thereby able to teach our kids the finer points of basic cycling while I was working on keeping everything balanced up front! It won't take long before they're capable of some fairly hectic stuff, which turns out to be fun both for dad and kids!
