Well i dont have and kids but i know all my sisters went on there first roller coaster when they had the chance. My little'est sister nikki went on the demon at PGA when she was barely even enough size to fit. She would get mad when we went on them and she couldent. I remember when i went on my first one when i was small and my dad made me go on it and i was scared of them all the way up until 4 years ago. Now i love them but it took at 8 years to get back on one! SO i think there is no age its just when THEY want to go in cause if u force them on they might be scared of them when they grow up.
Chris