I think that a lot of how long a helmet will last has to do with the type, i.e. full face or modular. I have four helmets at the moment, one full face, two modulars and one 3/4. My oldest helmet is a Nolan x-lite 1002, perhaps 6 years old. I think that this will be the last season for that helmet, as the latch mechanism is getting finicky, and the helmet is generally feeling a bit looser over the past year. I doubt that will be a problem with the Scorpion EXO-700 I own or the HJC Is-33, as there are less moving parts. Of course, the lining can wear out, but as long as a replacement is available you can buy a new one. But some helmets may be more worn out than others after 5 or more years of use. Modular flip up helmets would be more likely to be in this group due to more moving parts.
My other modular is a Shark Evoline V2. I've had it for 2 years, and given how much looser it has become (taking into account compression of the lining it has still become a bit loose) I doubt that I'd want to use it three more years down the road. But as with many things one doesn't know until one finds out.
I guess we'll have to meet back here in 3 or 4 years time and compare notes on the old helmets that we're still using, to see if 5 years is a good time to ditch them.
There are only two kinds of people who are really fascinating: people who know absolutely everything, and people who know absolutely nothing.