I have heard that for years and it holds true if there is damage when the helmet is dropped.
The exterior shell of the helmet rarely shows damage but that is where I begin to check for damage as in cracking or dents.
Where you can find 99% of the damage is with the compressed foam. If you see speckles or small pieces of foam inside the helmet you most likely have a damaged helmet and should replace it.
Remove the interior pieces of the helmet and check for cracks in the inner (exposed) layer of the foam. If you see speckles or small pieces of foam inside the helmet you most likely have a damaged helmet and should replace it.
The better helmets have another layer of compressed foam, or whatever they call it, that is bonded to the outer shell. You probably won't be able to see this layer but you might be able to hear it. If your helmet does not sound solid when you thump it with your hand then that layer is probably cracked and the helmets needs to be replaced.
Cabrito is correct and all of the helmet components will degrade over time and should be replaced.
Now finding that helmet that fits like your favorite.
I have never broke a motorcycle helmet though have broken two bicycle helmets. The first was lost to someon stepping on it. The second was lost to a downhill crash returning from Point Reyes Lighthouse. The first hit sounded like a 'pop'. The second had almost know sound other than 'ouch'.
Oh both Giro helmets were covered under warranty. Not in motorcycling.
x2 on checking the visor mounts to make sure they are not coming loose.
I've been told that over time helmets can become brittle and lose some of their protective qualities so they should be replaced periodically.
I also used to hear that if you drop your helmet that it could be damaged inside and it should be replaced. Even a small drop off your seat. I know in years past mine would occasionally fall off the bike with a big thunk on the ground, but I was younger then and would just keep wearing it.