IMHO, from an engineer's point of view, expiration dates on helmets and child car seats are BS. We don't have expiration dates on other plastic things in our cars. From an Occam's razor viewpoint, third parties have a financial interest in incentivizing people to replace their helmets and car seats under the guise of safety.
Maybe, just MAYBE, the foam on helmets can become weaker from UV exposure but there hasn't been scientific evidence for that. In fact, there was a study
disproving the claim that helmets become weaker with age.
https://asmedigitalcollection.asme.o...dFrom=fulltext
With all that said, if you have to glue a part of your helmet so it stays on your head, you should probably look into getting a new one. A helmet is supposed to stay on your head during a crash and if a strap is loose such that the helmet can rotate off, that's a bad thing.