Eight years ago I bought my 14 y/o QH gelding. I was getting back into horses at the time, as was a good friend of mine. She commented that she didn’t want a horse that needed maintenance, so she bought a 3 y/o. Which promptly went lame. Then got a 4 y/o, which again, had multiple issues and does to this day. Now has another young horse with multiple issues. Meanwhile, my old guy (now almost 23) is still trucking along. I got many years of stress free riding with him, as he is good minded and bombproof. He is now semi-retired but still does a few trail rides a month.
I think there is a lot to be said for a horse that has shown itself able to stay sound doing its intended job. There is also a lot to be said for a horse that has plenty of life experience - especially if the rider has confidence issues and/or is older (like me) and really, really doesn’t want to get hurt. Julie Goodnight’s current podcast addresses this very issue, and her advice to a rider who was relatively green was to buy a horse in the 10-12 year old range and stay away from the youngsters.
I am not a green rider, nor am I timid, but at age 64 I am risk averse. My newest horse is 8, and I only bought one that young because he is exceptionally quiet and good minded. Otherwise I would have been looking in the 10 and up age range.