What’s the age requirement to check in ?

You must have a government issued ID, so likely 18. You usually need a credit/debit card as well, but that can be pre-arranged. If you have a minor traveling alone, you might call the hotel and ask to speak to the manager. Explain the situation and maybe they'll offer a solution.

I'm not sure about that you would have to call the front desk. Most "high end" places it's 21 with a credit card for incidentals/deposit