Horses in Horses of Hoofprint Bay eventually retire and leave your active herd. The official FAQ confirms that outcome. It does not give a universal retirement age or describe a button to retire a horse on demand.
What retirement means for your yard
The official Horses of Hoofprint Bay FAQ describes retired horses as out of the game. If an older horse fills an important role in your stable, start planning which horse could take over its training or competition duties.
Read a horse’s age as in-game turns
The age shown on a horse is measured in in-game days, which are turns. Use it to compare horses when you consider a purchase. A number that looks old by real-life standards does not translate directly into a real-world age or tell you the exact number of turns left.
Keep retirement and neglect separate
The FAQ also describes animal welfare taking away neglected horses. That is a different outcome from aging into retirement. Keep up with normal care while the horse remains with you and plan for age-related changes separately.
FAQ
Can I retire a horse manually?
The official FAQ confirms that horses eventually retire. It does not describe a manual retirement control.
Does age mean years?
No. The game counts age in turns.
Can a retired horse stay on my active roster?
The developer describes a retired horse as out of the game.