Like fine wines, the good escorts get better with age. Assuming that a woman takes care of herself and does not put on significant extra weight, she can remain desirable for a long time. A lot of women reach their sexual peak after age 40. I guess you could call 40+ "mature."
I had my first "balls-to-the-wall-leaves-you-gasping-for-breath" experience with a mature woman who was (I thought) in her 40's and was surprised when she mentioned she was a Grandmother(!).
Just goes to show that older women can be hot and desirable (and a lot more "talented") in their "later" years.
As for me, when I picture "mature" I am usually thinking 35+.
This is my target demographic when looking for escort companionship.
I've discovered that the majority of under-30's out there today can't hold a conversation to save their life and when I can get them talking, I find we have absolutely nothing in common.