As noted above, helmets do not really degrade over time, unless you really knock 'em around or subject them to extreme heat.

Also keep in mind that helmets can't be recycled.

In my case, I used the same helmet for about 5 years, and just picked up a new one. I'll use the old one for commutes for as long as possible. I don't think the technology has improved significantly over the past 5 years.