I say what age are they
Sorry but to me that question does make one sound like a creep. You are going there for a massage, anything over and above that is only chance. Would you still ask questions like this if you are going to lets say a good spa only offering a legit massages? The massage establishment wants to run a genuine place, where even ladies for example can come for a massage. Believe it or not there are loads of people who just want to come for a massage and nothing else.
Humble advice next time book an appointment, just as you would book to meet, I don't know, your dentist!!! Once you are there and once you have built a little rapport, then you can ask their "age and sexual preference"
(to the masseuse not the dentist