This is from the FAQ on Helmets.org. Seems reasonable:

[h=3]An honest manufacturer: MET[/h]The Italian company MET says in their 2010 catalog:

"We are often asked 'For how long is a helmet safe?', or 'how often should I
replace my helmet?”' Until now it has been difficult to find any reliable
figures to help answer these queries. MET have now developed a series of tests
which are conducted on aged helmets to determine a 'best before' date (unless
the helmet is involved in an accident. In that case it should be replaced
immediately.). The results indicate that, if used properly accordingly to our
owner manual, our helmets will still do their job up to eight years after they
have been made. Not only is that good news for the customer, it’s great news for
the environment!"

We applaud MET for undertaking an actual testing program on helmet life and
for making that statement. We regard it as a triumph of integrity over
marketing. MET's helmets are made with industry standard shells and liners, so
there is no reason we can see that their recommendation should not be good for
many other helmet brands as well. If another manufacturer comes up with a
testing program that shows earlier deterioration in the protection from their
products we will review this page.
In sum, we don't find the case for replacing a helmet that meets the ASTM or
Snell standards that compelling if the helmet is still in good shape and fits
you well.