ATV's and dirtbikes get a bad rap, usually due to those folks who misuse them or do not take safety precautions. My son is a successful moto racer (age 7) and he does not even load his bike without all his gear on.
Constantly, when I tell people about his riding, their first question is, "do you make him wear a helmet?" And I always say, "No." Then I continue to say, "He wears a helmet, plus a chest protector, neck guard, elbow and knee guards and moto boots."
My plea to anyone who hears stories like this is to not think that any child is too young to be put on a motorized vehicle. Any age is fine as long as ear protection is present before age 3.