It varies with use, and with lots of variables both in level of use and exposure to sunlight (some evidence suggest that polystyrene deteriorates in sunlight)
The BHSI is an American organisation. It is "pro helmet" but is not afraid to tackle the issues and recognise the limitations and problems.
Read
this article - it is worthwhile to spend a few minutes reading this and the content is enlightening.
Only MET have tested older helmets and suggest 8 years, but Bell suggests 3, without performing any testing!