Your argument seems to be that we should not make generalised assumptions about the behaviour of minors based solely on their age, but to treat each case on an individual basis: to do otherwise, in your own word, is “discrimination” akin to racism. Thus a theatre excluding children because they have a perceived tendency to be noisy and restless is, according to you, “ageist”.
Of course it’s ageist. Society in its wisdom has decided that children lack the autonomous decision-making capacity, the maturity, of adults, and that certain decisions must be made for them, whether as a protection {not sending them out to sweep chimneys}, for their own benefit {sending them to school}, or the protection and benefit of society {not letting them drive}.
You seem to be arguing that this is a form of discrimination akin to racism, and that banning an individual from a theatre on the basis of his or her age is akin to banning them on the basis of his or her skin colour. To bar someone from a theatre because they are black is wrong because they are an adult human being capable of making decisions and being responsible for their behaviour: skin colour is not relevant to someone’s capacity to sit and watch a movie quietly. To bar someone from a theatre because they are five is simply recognising that five year olds as a class are likely to get bored and rowdy and piss off other patrons: their age is, by and large, relevant to their movie-watching capacity.
Honestly, you can’t have it both ways: if you’re arguing that minors ought to have the equivalent legal capacity of adults {“We have a responsibility to judge individuals as individuals, based on their own actions, not on their age, race, gender or any other accident of their birth”}, then you can’t on the other hand argue that age of consent laws are “to protect minors being tricked into doing something they don’t understand”.
Surely by your first argument a child ought to be able to make the individual decision to consent to sex themselves: fortunately, society recognises that both children and society itself do need protection in certain circumstances, and that protection is a direct consequence of a child’s age.
Since when is age an “accident of birth” anyway?