I tried to kill myself in middle school at 12 or 13 for reasons that I don't remember, but I failed and I forever regret it. I would still tell you to wait until you're 20 at least. I changed a lot when I was 19-20, after high school real adult life begins. If you are still the same by 20 then there's much less hope I think, but at 18 it's too early to tell even if you are KHHV. I'm only 4 years older but really, you're still a kid.
For me personally I haven't set any date, I don't plan on killing myself anymore because I should have done it in middle school but I lost my chance; it was the ideal time, I had no problems and no worries. If I'll still be an incel loser NEET at 25 I'll probably start doing insane things though, If you have nothing to lose why just kill yourself?