Which of the following is NOT an age classification for horses?

In the context of horse terminology, colts, fillies, and yearlings refer specifically to age classifications. A colt is a young male horse, typically under four years old, while a filly is a young female horse of similar age. A yearling is a horse that is one year old, so this term specifically denotes an age range as well.

However, gelding does not classify an age but denotes a horse that has been castrated, which is a procedure that can occur at various ages. Therefore, while it is a common term in the equine world, it does not pertain to the age classification of horses. This distinction makes it the correct choice as the term that does not fit within the age classification context.