Why is it that hotels ask for you to be 21 years or older to check in at a hotel??!!?

I am 20 and i was trying to make a reservation for a hotel near the beach :( but i cant since im only 20!!! and the other person thats going with me is 20 too!!! ahhh!!! what can i do!!!! all freakin hotels around there are 21+ :( aww man i wanted to go to a beach for the 4th of july :( :(