Good Age for ATV for Child?

My son wants an atv. I've looked up a little online for children riding ATV's but I really don't know anything about them personally.  I rode one once when I was about my son's age and I didn't have the strength to turn it.  I ran into a fence with it (good thing I wasn't heading towards the canal only a few feet away).  I could climb trees, jump fences, hit the top of the rope in gym, and run fast but I could not turn the handlebars on the four-wheeler. 

Anybody with experience want to chime in on ATV's for kids?