Personally, it depends what I want the horse to do and what it’s doing currently.
Pony club schoolmaster that’s been there and got the t-shirt? Absolutely would be interested in an older pony.
I think horses age quicker than ponies so there’s that to consider. Equally I need to decide if I’m in a position to be the horses last home… selling a late teens/early 20s horse doesn’t sit well with me… UNLESS they are currently doing the job well and able to school a less experienced rider at the job and therefore a more secure future.
I wouldn’t take on an older horse who wasn’t doing what I planned to do with them as I’d worry that the work would be too much- eg I was doing RC activities and some dressage to novice but hoping to go to elementary. So, for example, I wouldn’t be interested in a 16 year old 16.2 that’s mainly hacked as I’d worry that school work might aggravate arthritis etc that was as yet undiagnosed because hacking around the lanes was within its comfort zone. A 16 year old regularly competing at medium and doing well, happy to teach us the ropes would be different.
I’ve just started looking for a second. Because I’m looking for something bigger, 16.2 or so (due to sharing with lanky teen son), I wouldn’t want to go older than 11/12, unless it was the situation described above.
I think I’m probably influenced by personal experience. My cob of a lifetime had sought arthritic changes in his hocks at 13. He was medicated and carried on doing RC stuff but I was careful on hard ground, and didn’t go in the school more than 2/3times per week which was sometimes difficult in midwinter when lack of light limited hacking. Until he had a ligament injury (unrelated) last year, we had worked out a workload and management routine that suited him. But I knew it was on the cards that he might get worse and within a few years would be a happy hack… light hack… retired. We bought him age 7 and he owes us nothing so I wouldn’t mind… but i would worry if I bought a horse in its late teens that age related wear and tear might necessitate a long retirement within a year or two… I can’t afford three horses so I wouldn’t be able to ride anymore.