I think each must have her own very personal reasons.
I started at 27, and when I showed up the provider was stunned. She said "You're young, how old are you?" BUT, this was not asked until after round one, I think she was sizing me up. Then she offered to do blown and go's at a very discounted rate, if I wanted to be a regular. She seemed to be giving me the young man discount.
My second time, and the lady asked my age. I asked her, and she said much older than you. She looked fine, but definitely older than her pictures. She also lied about her level of service, that was my first time being robbed . . . oops, I mean YMMV situation.
Now you imagine that they see older dudes, and you imagine you get that...but you don't. Imagination is not sufficient. About 12-13 years into the hobby I had a mishap with an SD card from the camcorder, and I was given a recording of an earlier session. No shit, this chick was with a friend and they were double teaming The Crypt Keeper! Every square inch of this dude was wrinkled and those wrinkles had wrinkles! He must have been late 80's, covered in liver spots, 5 hairs left on his head, and everything sagged. Ears the size of saucers. And they were doing it with enthusiasm! Now, this first grossed me out, but, I'm not suckin his dick! And then I actually was happy about it, because when I'm 85 I want a session like that! And there are young girls that will do it! That gives me hope for the future!
But until you see it, you don't really appreciate the service. These girls act way better than you think, even when you think they've done nothing special and gave you average service, she was acting! And you think you're young, but at 40, I see now that I am still young too! I think most people under 60-70 are probably "young". But there does seem to be a 40 cuttoff in the hobby.
I did get asked a lot about my age, but as I got fatter, the questions stopped by 35, probably because I looked 40.
I think there are stamina and expectation issues from younger men. Also that question, why are you here . . . as dudes we have different answers. My answer was, I have a wife, I don't need companionship, she'll talk my ear off any time. I need a professional fucker with experience, and a hot body. Sounds like we are on the same path, but you may have noticed by now that Provider does not Porn Star. Even in the hobby, you still have to search for the gems!
Good luck on your journey. And consider yourself lucky that you found this outlet so early in life!