How old do you have to be to check-in to most hotels or motels?

I saw one hotel in Bethesda, Maryland that said you had to be 21 to check-in. I was wondering if that was the age for most hotels. If not what is? Is there a state or federal law sliema that makes the age requirement or is it the hotel itself that makes the rule. Any idea why? And are these rules flexible or negotiable, I am not talking bribery.