Life span of helmets

[Old fart]

What is the average life span of a helmet. Does it depend on KM travelled, exposure to sunlight,rain etc? Mine I've had for 4 years, never been dropped or crashed but I was told there is an expiry date on all helmets.
Your input will be appreciated.

 

[Slainte Mhaith]

 

[Mzee]

 

[StevedW]

 

[Spirit]

 

[Malibu]

 

[Adventurer]

 

[tau]

 

[HeftyCrab]

 

[Slainte Mhaith]

 

[Malibu]

 

[mox]

 

[Malibu]

 

[mox]

 

[Malibu]

 

[Mzee]

 

[Giepie]

 

[Malibu]

 

[DirtyTeNeReLess]

 

[mox]