DS got his trike when he was 18 months or so.
He just got his first bike this morning and he is 3.5 years. We got a 12 inch bike that coverts between a balance bike and a pedal bike (the pedals and chain is detachable). DS is tall for his age and he only just reaches the ground on tippy toes making me think that any younger would have been a bit too young. Unless you can get a smaller bike than that.