So, this isn’t as cut and dried as I’d hoped. But it sounds as if she’ll be OK if she has a credit card. Of course, I expect for the first few visit, the Worried Mom Mode will kick in and I’ll wind up sending her air fare.
Anyway, thanks, all, for the information.
It’s up to the individual hotel what their policy will be.
In the hotel I worked for, a franchise of large inter-national chain, enforcement of our 21-and over policy was extremely flexable. By policy, we were supposed to get a photocopy of anyone who looked younger. Rigidly enforced on Prom Night only, we were given discretion to decide whether to allow a younger person to rent. If they looked like they were the type to party, if there was more than one of them, or they tried to rent early in the evening, show them the door. If they’re obviously travelling, are alone, have more than one credit card in their own name, or just plain look like a “nice kid,” get a copy of their drivers license, and let them stay.
At first, we did take reservations if Mom called or came in and made it, but this was an absolute disaster.
Don’t bother. The matter of whether a minor should or should not travel alone is one of opinion or debate, not fact.
Accordingly all future posts about the matter, on either side, will go in another forum.
Been many years ago but I rented a motel room for my girl friend and myself when I was 17. They never asked how old I was… funny, I signed the register as Mr & Mrs… I was a little scared. 
My girlfriend recently got a hotel room (not what you’re thinking, she was traveling). She’s 18 and was not asked for any ID, she paid in cash and did not need a credit card. She had to sign in a registry, though.
Wow, two threads on the hotel industry on the top page of GQ, I can actually contribute something.
I work as a CSR for a corp that holds the franchising rights to 9 hotel chains and this question always comes up around spring break and prom time. As several posters have noted above, the minimum age for check in is generally left to the hotel to set. Some communities, such as around the Chicago area seemingly do have an ordinance that requires the guest be at least 18 years of age to rent a room but they are pretty rare.
From what I’ve seen, many smaller hotels don’t seem to care what the age of the guest is as long as they have the money to pay for the room.
The best thing to do is to contact the hotel prior to staying, of course this only works when you know where you are going to stay.
On a side note, if you are going to stay at a hotel and use someone else’s credit card or give the card to your child, etc…Please, for the love of Og, call ahead. Many hotels have different procedures for this and I can’t tell you how many calls I’ve gotten about this. (And all start out about how Mommy’s little 18-20 year boy/girl is being denied a room by the hotel and will have to sleep in their car.) Most times the hotel will require a faxed statement authorizing the use of the card and a copy, front and back, of the card holder’s driver license. This is to protect the hotel against charge backs and not because they are being difficult.