Personally I can_t remember ever being asked for an ID at a hotel. However I do look older than I am.
THE MAJORITY REQUIRE YOU TO BE AT LEAST 21 YEARS OLD, HOWEVER, SOME ALLOW 18 YEAR OLDS TO CHECK IN.
THERE ARE ZERO HOTELS THAT WILL ALLOW ANYONE UNDER 18 TO CHECK IN BY THEM SELVES, EVEN IF THEY HAVE A CREDIT CARD.
IF YOU WANT TO KNOW SOME HOTELS THAT ALLOW 18 YEAR OLDS, E-MAIL ME!
I think I stayed in hotels before I was 18, but I guess it depends on what type of hotel. I doubt a regular one would care. An expensive place would most likely check age/id