Helmet integrity

[GrgGmiller@aol.com]

Is it true that helmet's are only good for a couple years, then the 
structural integrity is lost?  
Is a bicycle helmet as strong as an equestrian helmet?......Do you really get 
what you pay for?  
IF it is true that helmets ARE disposable every couple years, how do we know 
the helmet off the shelf isn't already expired.....do they have expiration 
dates?  When I see helmets on sale I think they are old and need to be moved 
out of inventory.  
I have a 5 year old helmet and am concerned....would appreciate some advice.
Bonnie Miller
grggmiller@aol.com