I think luck is the biggest factor. I’ve had two over 30:
First was a medium Chincoteague pony, born wild in 1980, but rounded up as a foal. I got him when he was 3. He had at least 12 hours/day turnout and no special care (it was the 1980s-90s). I sold him at 12, he foundered badly at 18 and I took him back at 19, and he became sound enough to be my son’s first pony. He lived to 33. In his last few years, he had Cushings, and progressively had trouble first with hay, then even pasture, with more and more replaced with soaked but not soggy senior feed, beet pulp, hay pellet, and for a while, chopped forage. I fed him at least 4 times per day. He choked at least once a year and colicked a couple times each winter. Most would resolve on their own or with a short trailer ride. What did him in in the end was a tough to cure sinus infection which came back and required a few different antibiotics. It really made him frail and weak, and the second time he went down and couldn’t get up was his last.
My current horse is 31. I’ve known her since birth and owned her since she was 2. She’s a warmblood, so not a breed usually associated with living a long time. She’s turned out in a hilly pasture at least 12 hours per day. She has needed no changes to her care or routine until coming out of this winter. She has lost a ton of muscle. I’ve added senior feed and greatly increased how much she gets, plus the grass has come back. She’s only very slowly looking slightly better and I think I will probably have her put down before this coming winter. In the meantime, she seems happy, gets around okay, and has a good appetite.
There’s something to be said for the horses that only make it to 25, do well up until the end, then go out quickly. Managing an elderly horse can require a lot of extra work, extra cost, and extra worry. Clearly, I keep them around anyway, but every day I look at my horse with a critical eye and wonder when the time is right to let her go. I don’t want the guilt of doing it too early, or the guilt or consequences of doing it too late. I think “before winter” is a valid choice for her, but I need to be ready to make the call sooner if she goes downhill at all.