The winter compound will harden and perform worse in freezing temps. The overly soft compound is ok in summer for non agressive driving. If, however, they are 'all season tires', it is time to replace. If you see ANY checking between tread, or on sidewall, it is also time. 6 years is about my cutoff personally, 4 for winters stored out of the sun (for use in winter) and 2 years for r compounds/<200 treadwear tires.

These are all ballpark values I have found by experince, and hersay at the track and on the street. I personally find Blizzak and Nokian to fare best in the winter tire niche. They are both good snow/ice hybrid design now, and often used in rally/rallycross.