I too have started some older horses.
Rushcreek PDQ was started as a nine year old and was under saddle after one
afternoon in the round pen, and going down the dirt road alone in two
days.
My current horse, Bey Allure had five days
under saddle on her when I got her at age 7, at 8 she did 300 endurance miles
(all 50s) and at 9 she will be going to Tevis!
Marv is right, older mares are like older
woman.....we are just BETTER than we were as silly fillies!