My mare went from barely 15.1 at 3yrs old to a full 15.3 at 4.5 yrs. Even her head grew! When I acquired Belle last June, she wore a regular sized halter. Then this summer, her nose and on her halter started to rub on her cheeks and felt tight, and I could barely do up the halter around her ears! So now she wears a large horse size halter. I didn't think she would grow that much in a little over a year.