I think helmets have a life of around 5 years, personally, if they are being used frequently (meaning not sitting on your shelf cause you never ride) but the guy on helmets.org says some models from the 90s are still good for now. Some of it will be common sense... if you're thinking it's time, you're probably right.

I have a 94 helmet, and I keep it as a spare/backup which has come in handy a few times - it looks solid and has never been damaged. But I ride and race with a newer one (06) which has a locking back strap thing, and that just fits a lot better.

Of course the conventional wisdom is that you should replace it after any crash or other forceful impact to the helmet.