In reply to Indy:
The best way to protect the helmet pads from deterioration caused by sweat is to wear a buff, which acts as an evaporator in summer, keeping you cool and an insulator in winter keeping you warm. It also prevents direct contact with your bare scalp when that pesky wasp flies in the front vent. Rinse out the buff in the shower after your ride and everything stays nice and fresh.
A styrofoam helmet doesn't need to be structurally strong to protect your head; the idea is simply that there's an inch or two of foam between your skull and the road so as to absorb shock. AFAIK age doesn't reduce the already limited structural strength of a cycling helmet and I can't see how age would affect the shock-absorbancy of the styrofoam so I will continue to ignore the industry accepted wisdom about replacing my helmet and keep it for as long as it doesn't look too shabby. I've witnessed a few bike crashes and each time the helmet has done the shock absorbing job, in one crash of my own to the extent that I saw stars briefly and the texture of the ground was imprinted in the outer shell. The helmet broke along the middle so of course I replaced it as it wouldn't sit on my head right.