Do (all) hostels in the USA require guests to be 18+?

I’ve checked some hostel websites and in many descriptions, there was a requirement that guests be either 18+ or accompanied by a legal guardian.

Additionaly, if anyone knows for a great place (by great I mean a bed with linens, toilet + shower and Wi-Fi access) to stay in Boston and NYC, bring it on. I’ll probably be going to MA in the last week of October and am considering touring the NYC.