When can train baby to ride on a bicycle?

I started my son with cruzee balance bike since he is 1.5 years old. I yet to transfer him to 2 wheels bike as I want to wait till he is older, i.e. 3 years old. Good thing about started with balance bike is kid can skip using supported wheel when ride on bicycle as they have learned balance well. For my daugther, she can ride on bicycle since she is 4.5 years old, but with supported wheels. She can't ride on 2 wheels bicycle alone yet.