How old do you have to be to ride a dirt bike?

How old do you have to be to ride a dirt bike?
You need to be at least 4 or 5. A good bike for 4 and 5 year olds would be an 50cc. From 8 to 11 a jr80 or crf80 would be good. from 10, 11 and 12 something like crf100s or cr80s.