Ageism is discrimination based on age. We find ageism against the young acceptable on a wider level because adults are charged with the task of protecting children. That’s just how it rolls. We don’t let kids make choices about what movies they see unless their parents approve (which, btw, isn’t a law or anything, it’s a rule that movie theaters made up), we make them wait to drive until hopefully they’ve got some sense in their heads, the point of this ageism is to protect and educate.
And if you complain about that kind of ageism you must also be complaining that it is illegal for a consenting 12 year old to have sex with a consenting 50 year old. Because the only thing we find wrong about it is the ages involved. Unless you are willing to give that there is something about a 12 year old that means they can’t fully make those decisions. In which case there must be other decisions they can’t make.
The real problem with ageism in this country is discrimination against the elderly. It is almost impossible for them to get jobs, when they do it is the greeter at WalMart. Why? Because we’ve practically institutionalized a form of ageism that doesn’t take into account expanding life times, the improvements in mental health later in life, and other advances of medicine and science.