The deer is missing his first tooth or first premolar. Deer have 3 molars teeth 4, 5, and 6. And 3 premolars or milk teeth, teeth 1, 2, and 3. The premolars are replaced by permanent teeth between 1.5 and 2.5 years of age. If the 3rd tooth or in this case 2nd tooth has two cusps the deer is at least 2.5 years old. From birth until the 3rd tooth is replaced it will have three cusps. Based on the wear of the 1st and 2nd molars (the 3rd and 4th teeth of this deer) I'd say he's 6.5 maybe 7.5.