How old do you have to be to ride bike?

In the USA, riders can start racing 125GP bikes on full size tracks with WERA at age 10, and with the USGPRU (United States Grand Prix Riders Union) at age 12. In the UK and Spain you have to be 14 to ride 125s in the national series, BSB and CEV respectively. In Australia you have to be 16, which is why Casey Stoner went to the UK to continue his career in road racing.