Sorry for this generic answer but it is the truth.
When the snow gets deep enough to cover their feed up high to where they can''t get to it. It might start in October or December, it just depends on the year. I know of a herd in Utah that winters at 9500' + just because the wind blows the snow off of the high ridge that they live on and there is feed all winter long.
Like this last winter for example. Where I live here the winter was so mild I never saw any elk come down as usual. In general there are several hundred not far from the house by December/January but they never made it all this year.
As a Taxidermist I look at hundreds of Whitetail Bucks every year. I know that I have seen many different ways people judge the size of racks on live deer. I think for the most part they all are pretty close. I'm going to give you some measurements that I have noticed being right on in just about every deer. We have used it in the last couple years and have only been off by about 5-6 inches. We do study the deer while bowhunting so we get a little more time to look at them....