Breeding carries some risk at any age. I personally know of a couple of mares bred for the first time in their late teens, with vet approval (thorough reproductive exam prior to breeding) and minor supervision, who did just fine.
However all horses' bodies age, and there is some speculation that internal structures that have not been "stretched" by carrying and delivering a foal in younger years might be somewhat more prone to injury if "stretched" for the first time as an aged mare, when those structures could be thinner, more brittle, etc. This is not a firmly established fact, nor have I seen specific studies, and it probably depends alot on the individual.
That being said, broodmares that have carried and delivered before might also have damaged or weakened areas in their internal structures BECAUSE they have carried and foaled previously, that could be more easily injured or re-injured by a subsequent pregnancy/delivery -- and they are still aging inside and out, regardless of previous pregnancies--- so the risk difference between breeding an older mare who has carried and foaled and breeding an older maiden mare could end up being a wash.
Personally, I might consider breeding a maiden mare that was up in her late teens if she had been examined and cleared by my vet. I don't think I would mess with anything 20 or over. I doubt if its a magic number, it just sounds too old
