With aging horses, you have several options:

• Age only once per real day.
  
• Use an aging point.
• Use the Black Market item Morpheus' Arms to age horses for free until the age of 30.
  

Aging only once per real day is the entirely free option, and obviously the most financially sustainable.

Using an aging point, if paying for them by using passes... would result in 1 pass covering 10 years of a horse's life.

Morpheus' Arms costs 2 passes for 30 years of a horse's life.