So much depends on what breed the horse is, what kind of diet the horse has had, veterinary care, etc. I also believe that having one owner for a lifetime or very few has an impact on certain horses as well. There are emotional reasons for this, if you want to get airy fairy :) which I am to a degree, as well as the simple fact of consistency. Horses do well with consistency - diet, routine, home, person. I think that promotes good health as well.
I think for an average horse, that's had average care, maybe quite a few homes, perhaps a few dropped balls here and there nutritionally/teeth care/vet care wise along the way - twenties is average. For horses that have had exceptional and consistent care, your expectations of wellness into their thirties are not unfounded AT ALL!
I fully look forward to looking my guy in the eye when he's in his thirties. And knowing I did my best by him along the way. :) Cheers to that!