I wouldn't say it's a myth and just dismiss it like that, I mean, lids are plastic at the end of the day and plastic does break down and deteriorate due to exposure and UV rays over time so while most helmets, properly cared for, would last in excess of 5 years - would you really want to take the risk of one under-performing when you need it most? So many variables to take into account, I'd always feel better replacing than second guessing the condition. Have a read of this from a guy who knows pretty much everything you need to know about motorbike safety gear.. https://www.themotorbikeforum.co.uk/viewtopic.php?p=953836#p953836 But I still buy expensive helmets because I like the way they feel