None. Nearly every ATV has great big stickers that say, "No riders under 16".
I live in Alaska. A place where in many communities the ATV is the family car. Here kids learn to ride them at 7 years old - however - they carefully ride at 7 years old. Mostly because every kid knows someone in their town who died or cant walk anymore because they did not carefully ride their ATV.
I let my then 14 year old nephew from Boston ride one of my ATV's. We were not going very fast at all, going down a very big, open, clear, flat, nice dirt road and he hit a guard rail. There is not reason for him to have hit it - except stupidity. Instead of stopping to look at something he took his eyes off the road and watched it as he drove and slammed into the guard rail - on a clear sunny day.
Kids under 16 do not understand how quickly they can go from ride to accident. It's why we don't let them on it.
And FYI - he did not learn his lesson. When he turned 17 he got a new Camaro for his birthday - it lasted 2 weeks before he totaled it out into a tree. Instead of driving a nice Camaro now - he has a $900 beater.