It depends on state law and/or company policy. Call the hotel and see what they say, usually you must be 18 to make the reservation and check in, but in some places you must be 21 to check in. Just make sure you have someone at least 21 in your room and you'll be fine. If you do call the hotel take notes on who you talked to (name and posistion), date, and time of the call. If you're still not sure ask for someone in management to confirm their policy. Even better if someone can email you a copy of the check in policy or you can print it out from the hotel's website.
I'm a very young looking 20 something and they always scrutinize my ID when checking into a hotel. Was past 21 when I started going to cons so I don't have any first hand experience with any hotels with a "18 to reserve 21 to check in" policy.
2011 costume plans: Suzaku (Yami no Matsuei), Lyanna Stark (A Song of Ice and Fire/Game of Thrones), Rowena Ravenclaw (Harry Potter)