Yes, tread compounds do harden and loose grip as they age. The process is accelerated with exposure to UV light and heat cycles.

The best wet traction is usually available with summer high performance tires, but their traction rapidly degrades as temperatures approach freezing. All season tires have compounds that have a lower "glass transition temperature" so they have better grip at low temperatures, but they are not as good in high summer temperatures.

All season tires also have "sipes" (small slits) to provide more biting edges in snow. The Michelin MXV4 is an all-season tire as the name and tread design implies. Summer high performance tires have large grooves to dissipate water and large tread blocks for stabililty under high loading, but they lack sipes, and this plus the 30-40 degree glass transition temperature yields poor snow traction.

Duke