Well, "the rockies" is a long way north to south, so it depends on where you mean, but I think the short answer is yeah, it's too late to be in the mountains. Early September is ok, mid/late September plan on waiting out some storms, mid-October plan on being snowed on.
If you look at weather.com, and select locations you are interested in, you can find a seasonal average graph - here's one for Frisco -- as you can see, you'll have highs in the 50's and lows in the 20's overnight. And while that precip number looks small, some of that will be snow.
http://www.weather.com/outlook/recre...omnav_outdoors