As for when I get too old: there are tons and tons of escorts who stay in the business well into their 40s and even 50s. They don't get the sheer numbers that younger women do, but they tend to have a very stable client base and regular appointments. The ones who do well tend to be very active in the hobbyist community, though, and I'm not very fond of that, so I'd quit when that started to be an issue. I'd also quit if I began to have any kind of mental health issue caused by escorting, or if I wanted to have kids.
You might have the wrong idea about agencies. Agencies don't technically hire escorts, they represent us. You could say we hire them, considering that they get paid out of our rates, but it's a more symbiotic relationship than that. An agency might stop representing an escort if she wasn't getting enough appointments to pay for her advertising for whatever reason, including age, but there are plenty of escorts out there who don't use an agency and only see regulars.