Depending on the source, I've read to replace tires every 5-10 years. Basically the compound hardens, which is really bad for a winter tire as that's one of the main advantages over all-seasons; the compound stays soft and compliant in cold temperatures.
NOAA is predicting neutral cycle, possibly entering into La Niņa cycle next winter. So average, possibly below average temperatures for us. The past 2 years, we've been in the El Niņo cycle, which is warmer than average in our area.
Climate Prediction Center: ENSO Diagnostic Discussion