This is an email I got when inquiring about a helmet I bought new which was manufactured 1 1/2 years ago.

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Your e-mail has been forwarded to Feridax (1957) Ltd. We are the
official UK importer and distributor of Shoei motorcycle helmets.

Shoei safety helmets do not have an expiry or use by date and will not start
to deteriorate until you start using it. Shoei recommend that on  average a helmet if kept in excellent condition should be replaced at least every five years from the date of first use.

However, this is dependant on the condition of the helmet and how often it is used. If you use a helmet on a regular basis it will need to replaced more often than a helmet that is only used occasionally. The most important factor to  take into consideration when deciding if you need to replace a helmet is  fit.
An incorrectly fitted helmet will have an affect on how well it  performs during an accident.

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I had previously been told that a motorcycle helmet needed to be replaced 3 years from the date of manufacture, or 5 years from the date of manufacture depending on whether it was polycarbonate or fibreglass.