Arai recommend replacing your helmet 7 years after date of manufacture/5 years from date of purchase, so if your helmet is for example already 5 years old when you purchase it, you still have 5 years warranty from that date. 

Helmets can take 4-8 weeks to reach us in the UK , we then ship to UK dealers who can have the helmet in store for months or even years. Some helmets, such as the Race Carbon can even spend 6 months in manufacture!

There are no rules or regulations within the UK on how old a helmet can be when its sold, this is because the 7 year replacement advisory purely lies with Arai, and not with other helmet brands.