I can appreciate that people have preferences for younger providers. However, in 10 years of meeting SP's all over North America, I can safely say the ones in their 30s-40s do a LOT better financially then the younger ones. I've been DYING to reach my 30's since I was 20 for that very reason.
It's a shallow stereotype to assume men just want younger women. I think people who participate in boards have a disproportionate idea of their representation of the Hobbying population. They're a small but visible and often loud portion, but they do not make up the majority. Far more of the men NEVER post or read forums, some don't give a damn, some shudder at the thought. Interestingly, the ones who want a young woman tend to be the ones who post - probably a co-relating demographic. I'm being a bit tongue in cheek when I say this, but it makes sense that a man in his 50's who slept with a 20 year old would want to brag about it.
There are many many men who prefer to date age appropriate, especially when they're going out to dinner, to catch a show, or to travel with. They tend not to post on boards and they tend not to review, they're really not interested in the "community". I have always envied the success of these older women. I also notice they tend to get a disproportionate amount of young guys contact them who like the idea of having a sexual experience with an older woman.
I've met highly successful escorts in their 50's and met a few in their 60's. They were in amazingly good shape, looked elegant and beautiful (meaning they held up well as they aged). Some of the clients they saw have been regulars for 20+ years, and they were well taken care of by the men in their lives, so they really didn't NEED new bookings. Men left them money in their wills, men set up permanent monthly arrangements, they were quite financially comfortable.
So my answer: A woman should retire when she no longer enjoys being a provider. It has nothing to do with age.