All horses, regardless of their date of birth, change their age on January 1st. Terms that are used to describe the sex and age of a horse include:

Foal - The offspring of horses, either male or female, up to one year of age.
Colt - Male offspring, up to four years of age.
Filly - Female offspring, up to four years of age or until the first breeding.
Mare - Mature female horse over four years of age or younger if bred.
Yeld Mare - Mature unfoaled female horse - has never given birth or is barren.
Stallion - Male horse that has not been gelded (casterated).
Gelding - Male horse that has been casterated of any age.
Yearling - A horse that is one year old up to two years old.
Weanling - A foal that has recently been weaned and no longer has access to its mother's milk.
Aged Horse - Any horse of any sex, over 10 years of age.