I really don't think breed has much to do with longevity. Maybe statistically certain breeds live longer (and I have no idea if that is true) but with the horses I've owned my Arabians lived to 24 and 26. They died of colic. My Paint lived into his early 30's and had to be euthanized because his arthritis issues got so bad he didn't have a good quality of life. So while I always hear Arabians live longer, I really don't know if that is true. My poor guys didn't.
Then, my Mustang died of colic at 20. I was totally shocked and heart broken. I expected at least another 5 years of RIDING out of him, and hoped he would live to 30.
But that would never put me off of an older horse. They are so worth the chance of them dying. And there is no guarantee a young horse won't get sick or injured either. My friend had a 3 yr old that had to get colic surgery (he's alive and well now).
Oldies are worth their weight in gold! I have a 20 yr old Foxtrotter mare still going strong and doing 4 hr trail rides in the mountains.