If I want a mare to continue to be a broodmare into her late teens and 20's I make sure she is in good condition and excercised. I also keep her bred during her later teens and early 20's. At least a foal every other year.
Mares that age are sometimes harder to get into foal. I think that is where most of the problem lies with breeding an older mare.
I do not think the risk is much greater to an older mare compared to a younger mare. A risk that IMO is not that common for most mares.
I think that sometimes we as humans look at a mares age and compare it to human years. That is not reasonable.
If you have doubts take her to an experienced equine repro vet.
If I buy this mare that is where she is headed for the PPE. I am not spending money buying this mare if she has issues getting in foal next year. Shalom