We live on a diary farm and our boys really wanted motorbikes, so we told them that they had to be riding their bikes without training wheels before they where allowed to ride a motorbike. So they where both riding without trainers before 4.
It is a completly individual thing my DS3 turnes 3 this weekend and has got a 3 wheeler trike that he can ride and peddle but he LOVES his scooter and would prefer riding it.