First, Bob, because your posts are driving me nuts, 2 =/= two =/= too =/= to. You saved all the time of typing a single character twice in that post. Retarded. Please do us all a favor; use numbers to describe a quantity. That's what they're for. Words are ALWAYS spelled with letters, and substituting a numeral for a word makes you look stupid and makes your posts difficult to read. If you want to be taken seriously, you have to stop this.
On topic, I bought my helmet in March of '08 and have no intention of replacing it anytime soon. Granted, I don't put the miles on my bike that some of you do (my commute recently got almost 30% longer, so now I have a six-mile ride, in town, to work). When I did put more miles on my old bike, though, I was using a helmet I got with the bike when I bought it from a friend. He had bought the helmet new, he had the bike for about a year (lived in town, about two miles from work), and then I had it for about three years. By the end of that time, the helmet had nothing more than a few nicks in the finish from my kids messing with it.
I have a hard time with the idea of replacing a lightly used helmet at five years because someone at HJC said so, but if it sees daily use, I suppose it would be wise to consider the effects of UV radiation on the composite structure. Since these are highly variable, the manufacturers have to err on the side of caution and tell you to replace it earlier rather than later. Imagine the liability they would assume if they told you it was fine for ten years and it had lost 40% of its impact strength by year seven...nobody could ever prove it was a contributing factor to an injury or death, but whether it's for generating sales or to CYA, it's smart to throw out a number that's reasonable, but maybe a little on the low side.