Replacing bike helmets

I was in Evans looking at bikes for my 6 year old, and the salesman was encouraging me to replace his helmet as well, as it's about 2-3 years old. It's never had any knocks but apparently the foam/protection material can weaken over time.
I'd not heard that before - is he offering sound advice or just trying to sell me another helmet?
Thanks,
Josh
I'd not heard that before - is he offering sound advice or just trying to sell me another helmet?
Thanks,
Josh
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The shop are talking double round things.