Hitting old foam with a hammer so that it shears apart is dramatic, but that's not the kind of force that it sees in actual use, and not a scientific test.

Here's a study on old used bicycle helmets which use the same materials: https://pubmed.ncbi.nlm.nih.gov/26902784/

It found no difference in impact attenuation properties.

I don't know if they tell us that just to sell more helmets, but I'm fully convinced there is no such thing as a shelf life for a helmet. That's not to say there aren't good reasons for replacing a helmet in general, like wear and weathering to the shell / visor / pivot points, advances in features and tech. Personally I wouldn't buy a used helmet because it's tough to tell if it took a drop that could have compressed that foam. And yes there are people and curricula that will tell you a helmet lasts only 2-4 years, even the MSF: https://smarter-usa.org/wp-content/uploads/2017/06/26321_MSF.pdf I was a RiderCoach myself, and I think that is so dumb. Could you imagine paying $300+ every two years to replace a good helmet with no damage? That's more than than the depreciation my bike has taken while I've owned it. I'm interested to see the legal reasoning presented for compensation based on helmet age. To me that doesn't make a ton of sense, but a lot of questionable logic has factored into judgments in the past.