Untrue:
Motorcycle helmets do not expire if they are not used. There are many recommendations as to when to replace a motorcycle helmet and all are arbitrary and motorcycle helmets expire
agvsport.com
But if you need an excuse to buy a new lid, we'll all come up with reeeeally good excuses why you need a new helmet:
"Paint is fading on the old lid which makes it a safety concern cause cagers can't see me anymore"
"Inner liner is wearing thin causing the acoustic shielding properties to diminish so I'm at greater risk of contracting tinnitus. Or herpes. Whichever is the more serious one"
"Flew into a swarm of bees and the little buggers are stuck in the vents, which will make the bike fall down"
"It's Wednesday..."