Depending on breed, management and environment, the modern domestic horse has a life expectancy of 25 to 30 years. Uncommonly, a few animals live into their 40s and, occasionally, beyond. The oldest verifiable record was "Old Billy", a 19th-century horse that lived to the age of 62. In modern times, Sugar Puff, who had been listed in Guinness World Records as the world's oldest living pony, died in 2007 at age 56.

Regardless of a horse or pony's actual birth date, for most competition purposes a year is added to its age each January 1 of each year in the Northern Hemisphere and each August 1 in the Southern Hemisphere. The exception is in endurance riding, where the minimum age to compete is based on the animal's actual calendar age.

The following terminology is used to describe horses of various ages:

• Colt: A male horse under the age of four. A common terminology error is to call any young horse a "colt", when the term actually only refers to young male horses.
• Filly: A female horse under the age of four.
• Foal: A horse of either sex less than one year old. A nursing foal is sometimes called a suckling and a foal that has been weaned is called a weanling. Most domesticated foals are weaned at five to seven months of age, although foals can be weaned at four months with no adverse physical effects.
• Gelding: A castrated male horse of any age.
• Mare: A female horse four years old and older.
• Stallion: A non-castrated male horse four years old and older. The term "horse" is sometimes used colloquially to refer specifically to a stallion.
• Yearling: A horse of either sex that is between one and two years old.