We retired based on how the mare is doing body condition/mentality after foaling. Lady, 25 with 16 foals, Tina, 25 with 14 foals and Maria, 22 with 9 foals are the three eldest we have retired here. They were all forced into retirement a few years ago although they all are still cycling. I felt they had contributed enough to the genetic pool and deserved to just hang out! What I do which is a bit unorthodox is let all three of them keep their last daughter with them (self weaning) the foal. Lady and Tina weaned their girls between 8-14 months, but Maria, three years later keeps Patsy within eye sight and Patsy only stopped nursing when she was two! Interesting herd dynamic if a bit off topic!
I have retired mares at 16+ as I felt health or breeding wise they needed to be. The days of breeding anything that can because they can are long gone and a few of my early purchased while nice horses, really didn't need to keep contributing their genetics!
I have also purchased many a mare in the 15+ age range (including the three that are retired now) and if the fertility/healthy/breeding history supported it I wouldn't be afraid to buy another one.
Besides there is no guarantee on a 2, 3, or 4 year old becoming a good to excellent broodmare - that is a gamble too.