Yeah no, I'm in the same boat as you so I get it 100% I only pushed through because I saw some spoilers, but in the end I'll have to say I didn't regret it. The author didn't need the high school set up at all, but, alas... it is what it is ig. If you manage to push through volume 1, I have a feeling you won't regret it. Cheers!
Just to put the story in historical context, and you're free to not like the story btw because of it, this one story was specifically published at the start and peak of the Omegaverse genre in Japan. During this time, there was a lot of focus on SA and high-school aged characters (unsurprising for bl quite frankly). If you read the first volume, it's quite dramatic because that was the general popular style of stories (a traumatized Omega who is saved by their destined mate). It's evolved since, but it's origins are still from the wild west (as in everyone wrote everything no matter how fucked up) style of the early Omegaverse era. This explains why most didn't really care that the story began with a 17 and 20 year old being mates, because there were even worse things that were popular at the time. Personally it didn't bother me then and now because it's fiction (in real life I would oppose it) and because Hazuki was going to graduate soon anyways, so he was most likely going to turn 18 soon.
Ofc ofc, but I mean "not that old" more in the sense that this kind of topic was already becoming very sensitive as to media portrayals by then. But I'm not saying this in demerit of this manga; it's just that, unfortunately, it is what it is. It was an unfortunate and unnecessary choice from the author (who probably did know better, considering the following immediate timeskip), but we move on ig. I don't think we need to excuse a misstep in order to fully appreciate this as a good manga (which, it is, imo)
(on an unrelated sidenote, sorry to keep bumping this thread, OP)

his soulmate is a 17 year old high schooler?? correct me if I'm wrong or feel free to give me additional details, I'm literally on chapter 1 so I'm very puzzled.