That’s why the restaurant has an obligation to the other diners to insist on proper decorum in the restaurant. Restaurants throw out drunks who are disturbing other diners. Its an unpleasant task but a necessary one. The same should be true of any diners causing a disruption.
If a particular restaurant is having enough problems will unruly children I can see why they would have the temptation to institute age limits. It would be better to just quietly ask a group to leave but the restaurant gets to decide if there have been too many disruptive occasions with children and they are tired of dealing with it. I think its like throwing the baby out with the bathwater because many customer families are courteous and responsible diners.
The problem isn’t the children it’s parents who choose not to control them. My son works in Management in ‘High End establishments......generally speaking people who go there don’t bring unruly children... but he has also worked at Mid End establishments and often says the parents have no concern for others comfort while eating.....even when they approached about the matter.
Bad attitudes of parents today hurts everyone but they have little if any concern of that.