As you may know there are lots of variables to achieving and maintaining an erection that is useful in sexual penetration. That being said, these are the rules that my little head dictates, and that my big head interprets or justifies.
If she looks great, I will go maybe 5 years over my age, and that is if she is a remarkably preserved specimen! But I believe there is a hard line at 50. Let's face it, we ALL hit the wall eventually! One day people are telling you how young you look for your age, and then one day they aren't; when that day comes you have to move on to the Senior Tour.
I often find that older women have more skills, are willing to do freakier stuff, and are more worldly and can hold a conversation; ALL great qualities. BUT, I have found that the little head has told me very clearly that there is no substitute for youth. The skin just doesn't feel the same, there is a too soft feeling, and parts that should be soft and firm, are just soft or just firm . . . it sucks, but that's life. So, I generally have noted that the 50 is a number that almost all hit the wall, and there is no point rolling the dice, since I don't like the odds of failure.
I have picked up great sex techniques from the older providers (35-45) and they are wonderful, but at 40, I still prefer the younger girls. Maybe when I'm older, like 60-70, I"ll bump up to 45-50
And as an FYI, for those providers that do it, we guys know that the listed age on the websites are generally 5 years understated, and adjust accordingly. After all, you can only be 32 five times, before someone notices

So to my above statement, my cutoff is generally 35 (because that is 40), and I make exceptions based on reviews, menu, unusual level of preservation.