I'm looking for a new helmet, and I've seen a few on offer that are 2005/6 age, obviously they're new, unused & boxed etc. But is there any issue just with 'age' of a helmet?

I've just bought one much older than that so I believe not!

its UV exposure that "ages" the material.

If it's been stored out of the way properly it shouldnt have degraded substantially.

MET think their helmets last for about 8 years

How do met make their helmets

motorcycle helmet manufacturers (which use the same basic materials) usually recommend a lifespan of 5 years for a helmet in use, whether its been crashed or not, due to the damage caused by UV, so suggesting they have a shelf life too.

I dont replace my m/cycle or cycle lids that often and am always dubious of manufacturers recommendations – they have a vested interest in encouraging replacement. I would buy a new old-stock lid if it was suitably reduced in price – my 661 from CRC cost £39 and from the colour scheme I knew it was a few years old.

The real killers are sweat and as mentioned sunlight. If you use it a lot and you would like to rely on it helping you out when you need it change it every now and then.
Something stored in a shop should still be fine unless its been in the window.

I bought my xen in 05 its ferked… not its fault just used a lot, crashed in, de laminating cracked and definately fit for the bin.

BTW more on the subject in a thread I started last month
http://www.singletrackworld.com/forum/topic/new-helmet-time-what-should-i-be-looking-at

When inspecting the old helmet, before crushing it, I found there was quite a large crack in one of the main members, meaning it would have been almost useless in a good crash.

Checking is as important as remembering to replace.