Why do horses grow winter coats?

Horses usually mature at the age of 4-5. Often they reach full height at three, and then fill out when they are four and five, but it really varies depending on the horse.

This also depends on the breed of horse as some breeds like Trakhners mature very slowly and take time. So they can actually add more height as they get older. Thier are cases of some warmblood horses still growing in size until they are as much as 8-9 years old. Draft horses and any breeds that mature extremly slow can take a long while to mature and grow.

Many western horses (Quarter horses/Paints) are often full grown at 6 or 7 years old

In the U.K. horses can only get a Life Height certificate at the age of 7. Until that time they can get annual height certificates. Most horses/ponies continue to grow until they are 7 years old.