It takes a big hit on the helmet with your head in it to compress the inner styrofoam- yours should be fine.
Most people wear out a helmet from sunlight weakening the outer shell and heat inside the helmet starting to break down the foam, it starts to smell inside from sweat (Febreze spray helps), or they get tired of it.
The helmet companies want you to buy a new helmet every 5 years, probably a good idea if you have a Snell 2005 helmet because that standard was not very safe, but generally if you store your helmet out of sunlight and do not do any big crashes, most helmets will last at least 10 years, with at least one new face shield needed because of vehicles throwing sand and gravel off their tires. Dropping an empty helmet on the pavement rarely reduces the safety, just the appearance.
I think Arai will check their used helmets for just the shipping cost, if you want to be extra safe.