For the purpose of dating many motorcycle helmets, there is a simply production date sticker on your helmet, under the centre pad, stuck to the EPS (expanded polystyrene) inner. Some helmets have the year and month on the strap, but poke about a bit and you should find something in most.
For some, it will be about where your right ear sits in the helmet, under the padding. The first two numbers are the year. An Arai helmet is coded on the chin strap by the D-ring. It is either an easy date to work out or you can see the codes explained on the Arai support website.

Most bike helmet manufacturers recommend that a helmet should be replaced 5 years from date of purchase or 7 years from date of manufacture, whichever comes first.

This replacement recommendation is based on a consensus by both the helmet manufacturers and safety standards. Hair oils, body fluids and cosmetics, as well as normal "wear and tear" all contribute to helmet degradation. Petroleum based products present in cleaners, paints, fuels and other commonly encountered materials may also degrade materials used in many helmets possibly degrading performance. Don’t forget that the inside is made up of Polystyrene and we know that it just shrinks away when it comes into contact with the above.

A good tip is not to put your gloves inside your helmet. 70% of the crud thrown up that reaches our gloves is petroleum based (oil), or petrol itself when you fill up. You stuff your gloves inside your helmet and hey presto, contact with the EPS liner.

Additionally, experience indicates there will be a noticeable improvement in the protective characteristic of helmets over a five-year period due to advances in materials, designs, production methods and the standards. Thus, the recommendation for five-year helmet replacement is a judgment call stemming from a prudent safety philosophy.

For anything motorcycling related, please visit http://www.mymoto-uk.com  for more information and ranges of products that are available. This website stocks motorcycle helmets, motorcycle clothing, parts and accessories