just be calm at check-in, don't mention your age at all if for some reason they say "you're not old enough", ask to speak to a manager.....explain you have a credit card to guarantee the room and you don't want to cause any trouble and are tired and you just want some peace and quiet and would appreciate just being allowed to check-in also, since this is a priceline reservation, point out the fact that it's already paid for the hotel reserves the right to not allow you to check-in, the Bellagio 5* hotel in Las Vegas is a famous example of not being allowed to check-in unless you're 21