Generally, most children will try a two-wheeler with training wheels at around age 3. When picking a bike, your child should be able to sit on the seat and touch the ground, not on the toes, but with their flat feet. Not only does this size them for safety, but also to gives them a positive experience, building their confidence to eventually transition from training wheels.
Learning to Peddle
For the first timers, they may have trouble with the mechanics of pedaling. To help, put the training wheel on two catalogs or phone books so that the back tire spin freely. Help your child on the bike and placing your hands on their feet, help them pedal. In a very short time, they should be able to peddle without your help. Once you and they feel confortable with pedaling, remove the bike from books and have them try pedaling on a flat surface.