I'm wondering why you want to breed from her? Is it for sentimental reasons because she's a lovely mare and you want a baby from her ... ?
I personally wouldn't be inclined to breed from a mare over the age of 20 regardless of whether she's had a foal or not, I can only speculate that it wouldn't do her back, or pelvis any favours at all
A cousin of mine took to breeding horses a few years ago (goodness knows why) and I was told one of them was a 27 year old mare who was in foal - I nearly passed out! I've not heard since what happened to that mare or foal.