I would say give it a feel test on the helmet padding. Does it feel dry-rotted? Does it crumble under your firm thumb? Does it appear to be splitting anywhere? If so, then dump it, otherwise, I'd keep using it.

I just started commuting, so I haven't used mine "daily" for years, but I've only had my current helmet 2 years (maybe less) and I thought I saw some splitting in the padding the other day. It might be time for a new helmet for me too.