Experience tells us otherwise. Part of a parent's or guardian's job is to prevent harm to the (bratty) child. The (bratty) child may not like it, but that goes with the territory.
As we get older, our parents (seem to) get wiser.
Experience does not tell us otherwise. Certainly some 15 year olds are brats. So are some 5 year olds and some 25 year olds.
Some 15 year olds are fantastic human beings. Most are kinda average. They are not adults’ playthings to be mutilated and discarded.