Biologically, a deer is considered mature at 4 1/2. They are considered "young" as yearlings, and "middle-aged" at 2 1/2 through 3 1/2.

I do know some hunters that really can kill mature (4 1/2+) bucks almost every year, but 1) they are exceptionally good hunters who spend untold hours in a treestand; and/or 2) hunt areas with very low hunting pressure and a fair number of mature bucks in the area.

Due to the difficulty of seeing/killing mature bucks, I try to convince clients to just hold out for 3 1/2+ bucks. 3 1/2 year-old bucks are easier to see and kill than 4 1/2+ bucks, and they are generally carrying 85% of the antler size they will ever grow (so hunters aren't missing that much by taking a buck at 3 1/2).

I agree with BSK, 4.5 is fully mature but many 3.5's have the antler needed to make them a target. I try not shoot anything under 4.5 myself but under some circumstances if a 3.5 has enough antler to suit me he gets shot. As for the 6.5's and older becoming easier to kill I couldnt disagree more, the older bucks if anything move less and less during daylight as the years go by, and even react less and less to the rut!