Depends on his start in life a lot. I got Jemma as 20 months old. She was 13 - 13'1 the most (she was in bad situation, btw). Tiny... Now (she's little over 4) she's 14'3'' (and that's what all earlier string measurements shown she should be with full growth). No one believed she can pick up the height, but with good food, turn out and care she certainly did (and I didn't gave her supplements, just 12% grain with corn oil and hay, some grazing). Same with my paint - came as 20 months old as 13'2 - 13'3 now she's 15'1 and under 4 years old (still growing though).