Think about it this way: It make little sense to charge the same for a meal for a 3-4 year old as for a meal for an 11 year old.
Child meals can be considered loss-leaders - offerings that exist solely to make the dining experience more attractive to parents with young children, based on the premise that the vast majority of parents would be far too reticent about going to restaurants if they were expected to pay for a regular meal for their 3-4 year old. Viewed in that manner, child meals make a lot of sense, but the restaurant needs to determine under what conditions will their reason for offering child meals no longer be applicable - there needs to be a line drawn somewhere.
5 year olds? No - they eat only a scant amount more than 3-4 year olds, and so most parents would still be practically just as reticent about going to restaurants if they had to pay for a regular meal for their 5 year old. 6 year olds? Ditto. And so on. At some point, it gets to the point where parents are sufficiently "less reticent" about dining at the restaurant, that the offering of the loss-leader child meal is no longer serving its purpose. At that point, it just represents, on average, a loss to the restaurant.
That underlined word "parents" is important: This isn't about the child's behavior, but rather is all about the parent's (purchasing) behavior. It's about how the offering of the child meal for their child makes or breaks their decision to patronize the restaurant (versus walking out). And nothing ever depends on 100% of anything - it is just a matter of at what point enough of the parents are "sufficiently less reticent".
In the past, it's been pretty consistent that Disney restaurants would allow older children (and adults, for that matter) to order child meals at full-service (i.e., not buffet) restaurants. That may not always be the case.
Buffets are another story. There is no practical way of policing compliance with restriction to the child portion of buffet. Even if it was worth having CMs patrolling the tables making sure only people who paid for the full buffet are consuming items from the full buffet, it wouldn't be practical, because a CM calling a guest out for violating the rules is as likely as not to end up with the guest causing a scene or otherwise disrupting the restaurant until the full-price charge applied to them, no matter how justified, is dropped. Even though they're a small minority, there are enough exploiters and abusers that trusting guests to police themselves is a non-starter. Transgressors have ruined things for everyone, in that regard (and with regard to a lot of things in our society; not just this).
I wouldn't worry about your 11 year old not finding anything good on the regular menu/regular buffet. Generally there are choices available aimed at adults with non-adventurous palates, and if not Disney is typically very good at handling special requests for less adventurous food options at all their restaurants.
Have a great time.