I think it's really down to the individual temperament of the horse. I mean my girly is 17 this year, still acts like a baby and would be downright miserable if i tried to give her an easier life.
My Sunny was a riding school horse up to the age of 19 when I bought him. He was worked very hard and imo not fed appropriately coz he was "thin to the point of emaciation" (my vet's words) when I got him. He was so tired and sad and exhausted that the only way customers could get him into anything other than a shuffly trot was to beat the daylights out of him with the crop (I witnessed this one day and had to leave).
He's now 22/23 and he is a born-again ned! He is plump, happy, cheeky and a wonderful safe ride. He zooms everywhere with me, and is happy to canter or slop along, whatever I ask for. He'll pop jumps up to 2ft 9 and last year won me a first in walk-and-trot dressage! I've injured my knee at the moment so my electric-bummed friend is riding him twice a week for me - to keep him from getting too fat lol! And as soon as her backside hits his saddle, boy he's OFF! You honestly would NOT believe that he's the same, sad worn out ned that the riding school practically gave away.
So I would agree with PG and Ezme that it depends on the horse. These days, even 18 - 20 isn't really old as long as the horse gets the right feed, exercise and management. Oh, and is valued, respected and loved!
Yeah,totally depends on the individual horse- our oldest school pony is a wee 12.2 shetlandxhighland whos the grand old age of 29 and still totally loving her work! My own boy is 17 and enjoys his work still but is not really interested in the speedy stuff anymore and its a bit of an effort to make him move anywhere at speed. But,if hes had some time off, when you get back on him hes power walking everywhere and spooking at everything as if hes 5!! So not quite ready to retire yet!!
Our old boy is 25 and not intending to retire anytime soon thank you!! He hacks out two or three times a week, and my daughter still has a gallop on him. We had to retire him from jumping as he has no sight in one eye and very little in the other. He went to a local show on Monday and got second in the veteran horse class our 12.3 pony who is a baby at 19 got third, he still jumps and loves long fast hacks.
When/if they slow down really does depend on the horse, some seem to age very quickly if they retire, and are unhappy, others love it and thrive.