It can be a few reasons and each sex worker will have their own personal reason if they do or don’t. I won’t always refuse a young client but I do prefer older ones.
If they are a virgin. I would prefer not to be their first unless that’s what they really want to and have thought about it carefully.
Young customers are still learning the ‘hows’ of sex and aren’t always open to listening to the girl. There’s a tendency for them to think they just have to go as hard and fast for the whole time to be successful at sex and that can make it painful for the girl. Plus There is sooo much more to sex than that.
Also because they are still learning, they can ‘accidentally’ be rougher and not have the same finesse you get with practice. If they are willing to listen, that’s different but mostly they have a picture in their mind what is supposed to happen and they aren’t open to instruction.
Also if there is an age difference, it can feel a little weird.
They aren’t all like this but personally, it seems to happen more than not.