I was a very mature 12 year old (my mother gave me a credit card in my name at age 9 LOL). As mature as I was, I did NOT fully grasp all the long term effects of what I was doing.
All it sounds is like you are blaming kids and not understanding where they are coming from. If a kid starts going OTD at age 9, does that make it any better?
I think the reason that kids start in their teenage years is that they are beginning to learn more in depth and develop their sense of self much more than at age 8.
Does it really matter if they are halachically obligated in mitzvos or not? An 11 year old breaking halacha on purpose is tragic IMO and worse than a 15 year old. Maybe not in the technical halachic sense, but a young child that rebels often has much more problems than a 14 year old who doesn’t see the logic and beauty of Torah.