My two current helmets are about 4 years old and show no signs of degradation. They don't get very sweaty, since gravity, the pads, and my hair keep the sweat away from the helmet. I don't think the UV exposure on the foam is very severe, since I can't feel or see any difference.

I probably drop them once or twice a year but I am willing to bet my safety that they are fine, since there's no visible compression or cracking, or damage to the plastic shell. Dpending on how they look in a few years, I'll may replace them then-- but I'm really not convinced by the "new technology" argument. We're basically comparing molded styrofoam to molded styrofoam.