As a teen, I can tell you that teens don't need age-specific tournaments, but I know of very few (really only one ever) tournaments that have lower age limits (and the one was not at the TO's discretion but the venue's).
I say this because at least a few power rankings have teens on them (MN has a Melee Fox who is 14 at last check) and Nairo won Apex 2014 Brawl at 17 years old [taking first at one of the biggest Brawl events of the year].
So you [almost certainly] won't be barred from tournaments or anything, but I don't think you'll find tournaments only for people under 18 [and I don't think they're necessary either].
If you're worried it'll be two matches and out, well, that's what friendlies are for (and you could also try to attend tournaments with pools so that you'll get at least 3 sets in instead of two). And the more you go and practice, the more likely you'll improve and then be able to fight farther through bracket, thus making yourself a better player (and more able to enjoy Smash against strong opponents overall) and less likely to lose both opening matches.