In the southern part of the state I'd be surprised if 5% of bucks make it to be 5.5 years old. The hunters are too concentrated and between roadkills and habitat loss to urban sprawl the deer are being squeezed with ever growing pressure.

Drive north a couple of hours and the game changes completely. Deer dying of old age in the north woods is not uncommon. I wouldn't be surprised to see old deer 4.5-6.5 year old animals being killed with greater frequency by hunters.

In NH the highest percentage of bucks harvested over 3.5 years of age was in Coos Co.. The Great North Woods provides great hunting for older deer. I can attest that the hunting is more difficult then in the suburbs of the southern portion of the state.

So to actually answer your question, 3.5 years old is an older animal in the southern parts of NH and Maine. In the Northers or less populated portion of the state 3.5-5.5 is not uncommon.