The 12 inch size is the smallest and is usually suitable for 2 and 3 year olds as a first bike, although some older children of smaller stature could still use a 12". All 12" bikes will have training wheels (stabilisers) fitted, to assist with their balance while they learn the circular pedalling action. Most kids will grow out of a 12" bike before the bike is worn out, so they (the bike, not the child) can be passed around family and friends.