For good order sake,
1) Ask the girl to book the room to confirm she meets the age requirement.
2) Always prepare letter which will be signed by the girl where she declares her age as 18 or above. The customer will not be responsible if there are any documentary proof found otherwise.
3) Go for MILF as they provide quality(most of them) with their sexperience.
4) If all else fails, there is always abstinence.