That buck is without a doubt 3.5 and perhaps a bit older even. But 3.5 yr. old would just about be my best estimate.

A 2.5 yr. old will have much more of a squared head. A thin body with practically no sagging belly.

Neck size is not determined by age. Neck size develops as the breeding season draws nearer. Even a 1.5 yr. old will get a larger neck.