Depends what you mean by growing.
Growing in height and physical maturity are different.
Most horses' backbone is mature and solid at 6 years old. While some horses grow and change physically until 8,9 years old.
It depends on the breed mostly.
Most little breeds and pony end around 4-5ish but heavy draft or bigger boned horses wouldn't be done until 7-8 ish.
I know my draft cross gelding grew until 8. I bought him at 6 (16hand) and he is8 (16'3 hands now)
the more original breeds (mustang, Arabian ect.) stop around 4 years, and the more inbred (such as thoroughbred, warmbloods and even some quarter horses) don't stop until around 6 or so years old, from what I have heard and seen
Hi Carlie! As said previously horses reach their adult height around 5 years old, but this does not mean that their body and bones don't continue to grow, their global adult body is formed around 7 years old :)