I sympathize with OP's frustration. Though I am much older now I do remember being denied hotel rooms, rental cars, job opportunities, etc. at that age.

In my late teens and early 20s I did a lot of road trips with my own car. It required approval from absolutely nobody for me to hop in and drive wherever I pleased. I crashed with friends and relatives a lot to save money and because spending time with people was always part of the plan. Also, friends and relatives tend not to have 21+ policies about their guestrooms and couches. Though most hotels I've seen are only 18+; I believe 21+ occurs primarily in resort areas (esp. Spring Break destinations) in the US.