I'm not a breeder, but many years ago I remember seeing the grandmother (of my then Morgan gelding) who was 27 I think, and with a foal by her side! With that said, I believe she had been having foals somewhat continuously for the breeder.
One thing you might want to contemplate and discuss with your vet is the possibility of foundering during pregnancy. Apparently during some phases of pregnancy, there is a natural insulin resistance that occurs. Depending on her genetic predisposition to be an easy keeper (?) or the possibility of Cushings, which may begin manifesting itself in an older horse, this may be a risk, which known in advance can probably be managed via diet and exercise to avoid laminitis.
The reason I bring this up, is that we have a 21 year old Arab gelding at our barn right now with laminitis, although I believe that may have been triggered by his access to grass. My aforementioned Morgan gelding was a Cushings laminitic from age 26 until passing at age 31. Knowing what I know now, he was likely insulin resistant his entire life (being an easy keeper), but Cushings put him over the edge.
I now have a lovely Morgan mare that if I were younger, would love to have a baby from, but she is definitely an easy easy keeper and would probably be predisposed to laminitis without attentive management during pregnancy. She's not too old, I AM!
So, just a couple thoughts to ponder or Google!