It’s tough to guess without knowing the history of the horse. But a horse that’s been sturdy and sound (and especially having done so while in consistent work?), better, IMO, than a younger horse that hasn’t been in consistent work.
My 26yo mare is still going strong, and had she been bought at 12 could have done literally anything someone wanted until the age of maybe 24? Now at 26 she’s slowing down largely thanks to Cushings. But seems fair for a horse who had an incredible career for oh, 18 years of the time I’ve had her?
My 21yo OTTB started in the 1.40m jumpers at age 9 and showed from 1.40m - 1.55m from age 9-19. I think he could probably still be happily doing it if covid hadn’t hit and forced a break (and also impacted our ability to get to Canada where the only “local” 1.40+ classes exist). Now he’s packing my young teen daughter around 1.10m-1.15m. And I wouldn’t be surprised if they just keep climbing in height. He absolutely loves his job beyond belief. He is the stereotypical “iron horse.”
Years ago, a friend of mine bought a 17 or 18yo former GP horse for her daughter. That horse took her through the 1.40m, then when she graduated, carried her younger sister through the 1.40m after her. He must have been 23/24 by that point? Same story as my guys - he had always been a sound and sturdy horse.
So I think that buying an older horse is highly underrated. And if I trusted a seller to tell me that the horse has been sound and in consistently in work over a period of time I wouldn’t hesitate to buy one of that age. In fact, I would probably feel very preferential about a horse at that age who had shown that they held up versus a young horse. Reminds me of what people say about the “old” war horse OTTBs - to race for 5+ years and still be sound sure says a lot about the horse’s sturdiness!
I have one semi up for sale (not trying very hard, but don’t tell my DH that!) who is 12 and has won everything through the 1.40m, now packing my daughter around, and who hasn’t taken an off step in her life (knock on wood). It’s always so strange to me when people say, “oh, I just wish she was younger.” Well, she wouldn’t have the experience, nor would her ability to pack a kid around have been proven if she was?! But having said that, that’s more the normal craziness of the horse buying and selling world.
So I guess my summary would be that age isn’t “just a number,” but nor is it a defining factor. I think conformation and a consistent record matters to me more than age.