Once your horse has been put to bed for the night, you have to make him age. You can do this by waiting for the game to update, or by using aging points. When he wakes up, a new day begins for him. Horses age by a two month period. If you use the water of youth, your horse will age one month at a time instead of two.

To make a horse age without having to wait for the upgrade, you need aging points. These can be found during the horses actions (this is then indicated in the action history). Otherwise you can alsocredit you account with aging point by using a pass (5aging points for one pass) or buy the Morpheus's arms at the Black Market. Once given to a horse the Morpheus's arms allow you to age him as you like and as fast as you want without aging points or needing to wait. (Unlike the ageing points it can only be used on the horse it has been attributed to.)

If you don't have anymore aging points left, your horse will still automatically age during the upgrade (at 7am GMT). You should therefore think of putting him to bed before the night and before you log out of Howrse. Please note that if you have not made your horse do any actions, then he will not age at the upgrade. This means that when you do not play, your horse will not age.

At 25 years old, your horse slowly loses health as well as skills each time he ages.(the philosopher's stone protects the horse against these inconveniences). If you look after him well, he can live to a maximum age of 35, without a Philosopher's Stone.
-BM