The rule is to replace your motorcycle helmet every five years, sooner if you drop it or are involved in a crash. It is also important to remember its 5 years from the manufacture date not your purchase date.
You need to treat a helmet with care and store it in a cool, dry place. Be careful if you hang it on a hook as this can deform the interior foam.
I have been replacing my helmet every 3 years and have built up a collection. Keep checking back for some creative uses for old motorcycle helmets.
Helmet technology is progressing all the time and a new helmet is going to offer more protection than something old.