They will retire automatically once they reach a certain age.

What age would that be?

Iv had some of my horses peak at 17yrs so i normally retire them if im not using to breed at 19yrs old or i sell them at 18yrs old :)

They auto retire at about 30ish

I generally retire my horses at 30, or if they're linebred (unless I paid a ridiculous amount for them without being informed that they were linebred -_-).
The only time I'll retire foals is if they're below 34 stats, or they're linebred.

Okay. I have a 34 year old horse who isn't retired yet but I was looking to retire and a few in the late 20s I'm contemplating.

it really depends on you .

most the time a horse retires on its on sooner or latter unless you stop them

i try to only keep what i need/love

most of my horses are just for me .

but again its up to you what happens to your horses

I have had horses retire naturally at 35.

If you sell a horse these days you run the risk of othe players just retiring for you. There appears to be a few on here who buy just to hit the retire button. Not all are bad stats some are all tens and not line bred, so bit puzzled by this latest "craze"

I am like Elmo though, I keep what I like/need. I actually hate saying bye bye to a pixel esp if zip bought it with RL money bring a foundie. My New Forests ate just for me they won't ever be sold again.

I've had horses retire at 37 naturally, I was kind of sad when they did retire though, but I don't like keeping them alive.