Asexual as a term isn't new, it's been around since at least the early 1900s (so maybe a millennial label, but not the ones who are alive today). Like homosexuality, it's been pathologized, used as a reason to torture people, and generally associated with libido and kinks rather than (lack of) natural attraction as noticed in this thread.
Socially, humans name/label things for many reasons - to identify, to ease communication, to organize, to simplify, and - it has been said - to tame and to feel belonging. There are many mythos and cultural reasoning around the world that to name something is to take control of its power over you and even to take control of it. If it has a name others recognize, it means someone is not alone in that experience. It's why naming emotions is a powerful tool in emotional control. Normal things get labels just as much as abnormal things, arguable more so as many abnormal things we tend to just push aside as just weird if it's too out there for us to control.
We can debate some of that has gotten out of hand and that there are detrimental effects when said labels become the core of someone's sense of self - being controlled by it than using it to contain and label an experience -, but I struggle to see how having 4 basic sexualities (hetero-, homo-, bi-, a-) and treating them as traits like being able to label hair colour or one's interest in food (from utilitarian to foodie) to continue the great previous analogy.
The support needed for teenagers includes the sex ed to reinforce boundaries and consent and all the pp have said, support in feeling normal in a very sexualized world, and yes, these days, I'd argue in seeing it as a trait that doesn't need to define them. That's why, regardless of the sexuality that comes up, if a young person said 'I think I'm...', then I'd go "It's a possibility, we can talk about it if you like." or something similar in a positive but not like it's a big thing if they resolve differently later. I don't want my kids to fix their identity and sense of self around their sexuality, and I don't want them to feel that it's not up for discussion when they want or that they're broken.
Yeah, not having sex until in a loving relationship has been and in many spaces still is mainstream, but I still know people who cried in relief when told demisexuality has a name because they had felt broken. Sexuality had always been framed to them - as I think it still is even where the idea as mainstream - as something one holds back from expressing until that point in a relationship when they literally didn't feel any sort of sexual connection or attraction to anyone except 1-2 people after knowing them a very long time and were already platonically very close to them. They's dealt with a lot of shite because of not fancing anyone and percieved as cold, rude or oblivious to others interest in them which no one, of any sexuality, should have to put up with but it had left them feeling in the wrong. That label gives some people peace of mine that it's something that is real and normal part of the human experience and when it doesn't become an all-consuming part of someone's sense of self for an extended period of time (teens and others often temporarily move things into their sense of self when new and then it fades back out into a trait - like new PFB mums vs mums with adult kids), it doesn't do any harm that it would hurt kids to know about the asexuality spectrum.