Though helmets do not have an 'expiry date', it is good to change your helmet every 3-5 years, depending on your usage, maintenance, wear and tear of the helmet. As the materials used and several components of the helmet, Parts like inner lining, belts, EPS and polycarbonate shell will have gradual decay due to more usage or passing time. Some of the other reasons to consider for changing your helmet are:
If you had a crash or impact at speed of more than 80 Kmph. The damage to the helmet would be severe, even it doesn't appear externally.
If you have deep scratches on the decals; streaks that do not disappear with cleaning or rubbing.
If you have unglued air vents or damaged clips, frayed and worn straps, etc.
Apart from the above, some factors like prolonged exposure to UV rays, bad weather can also impact the life of the helmet. A well-maintained helmet that hasn’t seen a lot of action should probably be replaced after a maximum of five years of ownership; as safety should not be compromised.