Yes, I have seen this repeatedly. One was my trainer’s first horse, who insisted on being a lesson horse until the ripe old age of 32. He was getting arthritic by his mid twenties, but every attempt to retire him resulted in a miserable, cranky horse - paradoxically, he also got more uncomfortable because he wasn’t using himself properly. When put back in “work” (light wtc lessons with the occasional crossrail) he began to bloom again and was much happier and sounder. He finally consented to be retired at 32 when his arthritis went from “I’m stiff and can come out sore but I still LOOOOVE to work” to “Ouch, I would rather not do anything today”. I’m convinced that he would have passed far earlier had he not been kept in work - he lived to get ridden and adored all of the students. He passed about 9 months after his retirement, at 33.
My old man is now 20 and shows signs of the same kind of mentality; he has stepped down from being a 3’ show horse to teaching wtc thru the occasional 2’6 course due to arthritis, and we maintain him pretty aggressively for a lesson horse (equioxx, adequan 2x a year, will inject as needed). We tried to reduce his work this fall and he was visibly upset by it, cranky on the ground, and HYPER under saddle. We upped his work again and now he’s happy as as clam. He’s the type that LOVES to get ridden (especially to canter or jump, being a thoroughbred) and I doubt he will retire any time soon - he is still waiting at the gate in his paddock for me or one of his riders to come and get him and gets seriously peeved with me if I haven’t ridden him in a while. For what it’s worth, I still love on him and groom him plenty - I truly think that he believes that I am “his” person and thus I SHOULD ride him as that is job. He loves all the kids but is not particularly picky about who rides him, although he has an older lady who he has become incredibly attached to - again, if she doesn’t ride him “enough” in his eyes (even if he’s not sound!) he sulks and gets terribly cranky. We keep the jumps small with him at this point but he really thrives going about an hour a day 6 days a week - any less and he is a ball of energy. There are still days where he wants to work SO much that he will go twice, although he let him take the lead and tell us how he is feeling. We regularly get comments that there is no way this horse is 20!
That being said, I have met ponies that happily retired at 16 or so (accidentally, after a period of not having a rider) and stated their commitment to the lifestyle by strategically dumping everyone who tried bring them back into work, refusing to be caught in the field, etc. It really depends on the horse - I think they will tell you what they are up for. I think that being consistently fit, having good farrier work and vetrinary care, and consistently high quality nutrition (especially a LOT of fiber - our horses get basically all the hay they can eat, so long as they don’t get fat) really improves their longevity.
ETA: All the horses I mentioned had ample turnout (the 20 year old lives outside with a shed by choice, the others got at least 8 hours of turnout a day) and didn’t have any significant lifestyle changes in retirement OTHER than riding (i.e. same barn, same herd, same people, same amount of human interaction).
