There's one other difference between the 19 and 91 year olds that you are overlooking. It's quite obvious and straightforward why the 91-year old is hiring a sex worker. However, unless he has 24 hours of shore leave before the Navy makes him get back on the boat, the 19-year old is inherently far more sketchy.
He's in the prime years of his life for casual sex, and attracting mates. Why does he not have any friends with benefits? Why does he not have any exes, who departed on good enough terms for occasional hook-ups? Why does he not simply have a girlfriend?
The old guy is a known commodity. The young incel is a walking red flag, who even in the best-case scenario is comparatively poor and apparently wants to haggle you on your rates. I suspect that for the sex worker it all balances out well enough.
Stay yummy.