My personal trainer asked me this today. He's getting on a bit, he's finding the admin side of running a small gym/studio takes up too much of his time and the gym that he leases is either going to have a dramatic rent increase or be sold for redevelopment in the near future so he's thinking about options.
I recommended that he go into sex work as a dom, because he's a natural sadist and has a great torso (I haven't actually seen it in person as he wears baggy layers, but I've seen photos of him competing). We had a speculative conversation about just how revealing his clothing would have to be, and we did establish that PVC was nasty and leather was the way to go, but his heart wasn't in it. 
Now that I think about it, you don't really see older PTs, like over 50. Perhaps they are all so fit that they look late 30s up until they retire. I don't know if there's a natural career progression.