Check her teeth. Does she have six teeth top and bottom? If she does she is around a year. If she has 4 top and bottom she is 6-9 months. She is pretty I love mustangs I used to have one he was the most hard working horse I ever owned.
It's a string test. We did it on her and it measured 15 inches but not sure how accurate it is. You measure from the coronet to the middle of knee on the foreleg. We measured our 12 yr old mare to 14 inches and she is 14.1 hh. Only time will tell:)