Why not Hawaii? You can fly to a place like Kauai and spend a few days hiking then fly to HNL to party. I've been there when I was younger than 21 and never had any problem checking into a hotel or even ordering alcohol.

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.

Yep. Normally if you see a 21+ age restriction it's usually because one too many people in the 18-20 age group did something stupid (typically involving damage to property or people) and so the entire age group gets punished. So the best bet is to look for places off the beaten path for the normal 18-20 year old in the US, avoid popular spring break spots, etc.

I do not remember the last time I had to show an id at a hotel in the US/Canada. Usually they only ask for my credit card... Unless the OP looks like a 14yo, I doubt they will ask for his id.

I've seen a lot of hotels in the US require the person checking in to be 21+ and do ask for ID. Hostels don't seem to have that age requirement.

I'm in agreement that destination outside of the US would be the OP's best bet unless they can find some 18+ hotels in PR, especially if the OP plans to drink on vacation.

Cruise ships based on the US, regardless of where they're flagged, generally require one to be 21+ or married. The ship's country of registration has more to do with legal requirements surrounding the ship's operation.

I too used to travel a fair bit on my own between 17-21 and stayed at a wide range of low to high end hotels, and never got hassled on account of age. I get the impression that the age restriction thing is probably more of a handy excuse if they get a bad feeling about you... if you look mature and responsible and unlikely to cause a ruckus, I doubt they'd turn down your money. Even better if you have a credit card.

As noted by some of the other posters, this may not apply to popular young-person destinations (e.g. Daytona Beach at spring break) where some places may rigidly enforce age minimums.

EDIT: Given that it has been over a decade since I was 21, I defer to Tide_from_PAE who says they apparently do ID now. That's a surprise to me... no one ever cared when I showed up at the Fairmont when I was 17.