I own a 30 yr old right now. Besides a "joint" supplement (basically just to take the edge of pain off in the winter) she has very little extra care. She is probably more arthritic than this guy and due to some pretty major scarring from an accident last spring isn't rideable at all.
In terms of "extra" expenses you need to keep an eye on.
- pain relief: light bute, maybe some sort of pain relief supplement (that's what I'm using, although they call it a joint supplement)
- 2x yearly teeth check ups: important to catch anything right away due to the difficulty of putting weight on older horses
- better quality hay, possibly an alfalfa mix (check for Cushings and other metabolic issues first)
- initial vet work up: If I were to get an older horse I would probably do some initial testing for Cushings. If you know they have a slight issue a lot of times you can prevent it from getting worse by simple and cheap management. It's when you let it get really bad that it gets expensive to fix.
-Senior feed: This isn't a for sure thing. A lot of senior horses don't need it and it can aggravate any metabolic issues. Usually they need it when their teeth just can't do the job anymore. My mare isn't on anything other than hay and a ration balancer... the problem is keeping her from getting too fat!
Besides the aspect of him being calm and an ex-theraputic riding horse in my experience older horses are great because they usually really appreciate ANY attention you give them. I've had a 27 yo, 24 yo, and now my 30 yo and all of them just loved any sort of petting, grooming, etc. I'd go out in the pasture and they'd come ambling up just to hang out. A lot of them are just really peaceful (in my experience) and relaxing to be around. I really like older horses. I might even look into getting another oldie when Flame passes on to keep Soda company.