The problem with breeding an older mare is not the stress to an aging body, it is getting her in foal.
Mares experience very little stress on their joints from a pregnancy. Riding her would be more stressful and if fed a joint supplement I forsee no problems. The foal does not grow very much until the last trimester then it gains about a pound a day. I have a mare that foaled at 27 she is now 32 and thriving.
I have bred numerous mares in their 20's and late teens.
The oldest maiden has been 19 or 20 I have to go look at my records to be sure. She is now 23 and being ridden everyday by her owners children
If you can get her in foal then good nutrition is mandatory especially for a senior mare.
Contact an equine repro vet as you should have her tested for any infections prior to breeding her.
They are the experts in this field.
Good luck and which stallion are you considering? Shalom