Considering the galvaynes groove used to determine age is on the corner incisor (not ALL incisors) and we can't see the corner incisor, I don't think this picture accurately allows any real guess at age. This horse could be anywhere from 10 to 20 from what we can see here. Not to mention the groove is not very accurate and you have to look at other indicators like wear surface, presence of cups, tooth shape etc to get a good guess. Need more info.
Heres a good article Galvayne's Groove In Horses: Photos