Definitely an interesting ponder for me - this started as a kind of 'tide-me-over' thing for me but I genuinely enjoy it. If, when and how I either continue or 'retire' I think will in part depend on what the other components of my life look like (more in terms of time, responsibilities, commitments than financial considerations) at any given time so I suppose might not ever be absolute (which I think must be true of others having seen posts of those who've left for a while then come back to it).
The age thing doesn't influence me so much in terms of the number or perception, but if I end up with arthritis, glaucoma, varicose veins and mostly deaf I'm probably not going to feel particularly enthusiastic about continuing sex work (then again, who knows?!)