16-18 at minimum.
An 85cc bike is WAY to small for any 16-18 year old. Most 16 year old racers I know race in the 125cc 2t/ 250cc 4t class. And by the time you hit 18 you start to see alot of kids that age enter the 250cc 2t/ 450cc 4t class.
I would say around the age of 10 if you want to race in the 85cc class.