I bought a horse that is described as a school master, aged 17. Well, he's not 17 but closer to 20. I have had him for just about 8 months, and we have such a great bond. He is an amazing horse. Calm, sensitive, trained, responsive. My barn owner says she wishes she had more horses like him. Yes, I know he may only have another 5-8 years of moderate level activity, but for a first time horse owner, he is a perfect fit. There are no soundness issues, and he gets the best of care, chiro, supplements, vet, farrier, trainer and quality feed.
At my barn, there are many younger, green horses. Their owners are experienced, many of them my age (58 ish). Several of these people don't dare go riding on the trail by themselves because their horse isn't calm enough. They are limited in scope by their horse's green-ness. They must be lunged before riding. They're spooky, hot, etc. I am really glad I bought my guy. Initially I was against buying such an older horse, but the years I have with him are priceless. He will have big shoes to fill when or if I have to buy another horse!