I don't know, my old buckskin definitely "mellowed" at about 25-ish. Although he was a fantastic horse, I never put a beginner on him unless it was a very controlled situation. He was never "patient" w adult humans or dogs - but he was very "paternal" w small children and puppies. I figure seniors that are good beginner horses probably were for the majority of their lives.
I just had to include my mare in this thread. Sissy is my 28 year old AQHA mare that I bought when I was 9(she was 18 then). The first time I ever rode her, I hated her. Lol She was being used as a lesson horse at the barn I was taking lessons at. I soon fell in love with her and had to buy her. I was very inexperienced and she knew it. She would sometimes take advantage of me and other times she was an angel. To this day she is still that way. But she is the best horse I have ever had the privilege of knowing.
When I was younger I would walk out into the pasture with her and she would keep all the other horses away from me. She still does it. I can lead her around the barn without a halter. We can do a liberty showmanship routine. And she can honestly tell whenever I need cheering up.
She is also one of the best movers I have ever ridden. Just last year I still showed her and she placed above horses less than half her age! I wouldn't trade her for anything.
My kids' first horse was a 27 year old QH mare. I got her from one of oldest daughter's teachers and traded chores (roofing, fencing, feeding, cleaning up the property) for the horse and short term board while I finished college. The kids had a great time grooming her and taking short rides so they could each have turns. I loved being outside together and the work was a wonderful break from sitting in lectures and studying.
After I graduated and went back to ranching, this mare became a favorite at brandings and for any doctoring situations. She didn't have speed but was a great partner as she could "read" a cow or calf as well as anybody and stayed very calm. No wasted movement. Lots of young people roped their first calf in the branding corrals off her.
We kept her into her early thirties until too many of her teeth were gone and I put her down when I saw signs of her losing weight. There was no way I was going to let her slowly waste away and become defenseless against predators.
The first horse I ever rode was a 24 year old bay polish arabian, Dreamer! She was perfect for me, and tolerated my TERRIBLE riding (Since then I have been getting professional english lessons)
I still love riding her occasionally, even though my 5' 7" self looks a bit odd on her 14.2-3 body! She's fun, spunky, and I've never fallen off of her! I loev getting on her back and knowing that nothing could possibly go wrong.
This is my oldest girl, who will be turning 26 this year. Old, but still a nutbar. She's travelled from New York to Arizona showing and is my only spooky horse. LOL
But she's sweet as can be. I got her five (six?) years ago when her owners couldn't care for her, and offered her a retirement home. She is uber sweet. :)
The other old nag, Lady, also retired at 25.
And Snowy, at 20.
And I have to add Que, a mare I bought out of the kill pen at 23, who was one of the sweetest horses I've ever had the joy of owning. I lost her at 27.
The oldies can definitely still go strong! My mare is somewhere in her mid to late 30's and I ride her a couple of times a week. She has more energy than any other horse I've ridden and she knows so much. I started riding Lizzie about 9 years ago, when my riding teacher owned her. I like to say (and think it's true!) that Liz taught me more about riding than any human. Liz is as hyper as can be, but she will do anything to keep me safe. She's one of those horses who will throw anything at rider, but only if they can handle it. When I get on, it's all run run run. However, I can put little kids who have never been on a horse on her and she'll walk around for an hour, ignoring their kicks and turning whichever way their eyes look.
You may need to do more maintenance than with a younger horse. For example, if Liz doesn't get her joint supplements, she is uncomfortable. And sometimes, she's stiff if she's been standing around and needs to walk to loosen. Also, the oldies tend to do better with longer warm ups and cool downs. It's all manageable, though, and what they have to teach us is invaluable!
My first horse was 32 when I got him. He passed away quite a few years ago at the age of 42. He had been giving lessons 4 times a week until a week before he passed away.
Odie will turn 22 in May. I've had him since a coming 3 yr old. He has mellowed some, but is basically still the same knucklehead he always has been. He falls into the "crabby old man" catagory. He was great with J when she was first learning I.e. Didn't have a clue. Now that she's taking lessons = he has to actually work, he's starting up with his old tricks.