The "shelf life" of a helmet is VERY long. While sun and UV lights do
degrade a helmet, this degradation is very slow and only effects the
surface layers. Over time the foam does change it's characteristics but
that changes appear to be a wash as far as protection ability of the
helmet.

That being said a helmet offers almost no protection. Do NOT be fooled
into thinking that a helmet will protect your head in all but the very
slightest of accidents.

While 90% of all accidents fall into that catagory you should probably
be aware that just falling over and hitting your head against a curb
will substantially exceed the protective capacity of a helmet.

Wear a helmet if you prefer but don't let it take the place of
intelligent riding. Don't run stop signs. Always ride on the correct
side of the road. Remember that riding with others is perhaps the most
dangerous part of a ride since touching wheels will take you down
faster than anything else.