I 100% agree with the previous posters.
I have a 27 year old mare who, though she's finally physically starting to give into age a bit, mentally think she's about 4 1/2.
The things you have to think about with an older horse are:
1) they are yours until they die. After they hit 24-25, it's going to be hard, if not impossible to sell that horse to pretty much any reputable owner and by that point, the horse has really earned a stable reliable life anyway.
2) You're just in time (maybe in a few years in this gals case) for all the age-related expenses. Sure, young horses have unexpected expenses, but in old age, horses often get things like Cushings which require spendy daily medication. Or, she could be like my mare, made it all the way to 27 without issues but was just diagnosed with Moon Blindness in both eyes. That means she cannot go without a flymask when it's sunny, ever. If she has another inevitable Uveitis attack, it'll cost me about $175 just for that time. Then, eventually it'll happen again, etc etc. Then, if she ever needs surgery to remove her eyes (hopefully not), I'm looking a bill of over $300.
My girl would have passed a PPE 4 years ago when I got her and probably still would, except for the fact that she has pretty limited vision. When I bought her, I never even considered blindess as a possibility, ever. I thought she might become old and lame in her thirties (hope not, obviously) but I never considered that she might be looking at spending her thirties without sight...
All that being said, I love my older horse, she's a ton of fun. She's taught me so much and she has a home, no matter what, rideable/blind as a bat/whatever.
There's just a a lot more work and money, not to mention love, that goes into the responsible ownership of an older horses than is "required" for responsible ownership of a younger horse.
Fabio - 13 year old Arabian/Lipizzan gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.