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I have tried to see if I could get a testing organization to test an older helmet of mine to try to settle an argument, but I got no response. I have a feeling that a 10 year old helmet will still be about as effective as when new, but I have friends who disagree.

It looks like, at least for bicycle helmets, there is no significant degradation over time.

OTOH, standards can improve. Motorcycle helmets seem to do better than bike helmets for standards being changed over time.

For the last 40+ years, bike helmets have been designed to protect the contents of the head from a single direct impact, They are the best protection for the occasional crash when the cyclist is thrown over the handlebars such that he drops head first onto the pavement from a height of 5 to 6 feet, or less. That kind of crash doesn't happen very often.

Well, there are a few standards. Here's a link

Note that in most crashes, the cyclist, if they strike their head on the ground, will have an impact consistent with a drop of 5 to 6 feet. The cyclist may be going 20 mph, but their head is still likely to receive the same force as just dropping at 0 mph (since there's horizontal velocity (their speed)) and vertical velocity (force of gravity).

Now motorcycle helmets have a similar standard for an impact test (DOT, Snell, and ECE). Again, because despite your velocity in the horizontal plane, it's the same force of gravity. Motorcycle helmet standards tend to have a few other tests as well (such as making sure the visor doesn't crack if a rock hits it at 55 mph), but there's only so much that can be prevented by a helmet.

Now there are tradeoffs - you can make a helmet that's stiffer, or that survives multiple impacts better, but that tends to come at the cost of how much energy it can absorb in one impact. Snell faced accusations of this in one of their previous motorcycle helmet standards (either M2005 or M2010) - the helmet was more durable, but at the potential cost of more G-forces transmitted to the brain.

Nothing in the above should be interpreted as me being anti-helmet - I always wear bicycle helmets, as well as full face, either Snell or ECE rated, motorcycle helmets.

https://helmets.org/airbag.htm

A neat idea, but I don't know if they're ready for prime time.

Helmets aren't especially useful and at least some research say, they increase risk to the bicyclist's health.

Given how much more fun it is to ride without one, you may want to reconsider — unless you wear it all the time, even when walking. Just in case a car hits you...

Dunno about jetpacks, but bicycles just aren't fast enough for helmets to perceptibly increase one's chances in a rare accident to justify constantly incurring costs in comfort and situation-awareness during the rest of your riding. Yes, there are statistics showing correlation between fatalities and riding without helmet, but that does not prove causation.

Surely, everyone is entitled to making their own choices, and I'm not going to force anyone to ride without the protection they want. I just want the same freedom for myself.

Of course you can't do that, but you could do reasonable tests on a helmet of the same type to show that it might require a certain amount of acceleration to break, and it would show whether exceeding its failure threshold is still likely to bring an impact to within a survivable range. All recent helmet certifications have a minimum failure limit of 300g, so it should take more than that to actually break, but since an unprotected impact is likely to exceed 1000g it's not unreasonable to say that a broken helmet could well have saved you, perhaps by lowering the impact to 400g - that's not a good number, but it's quite definitely less bad. Anything that increases impact time (and thus reduces acceleration) is a good thing.

Head injuries aren't rare in serious bike accidents. In fatalities, 2/3 had head injuries. Fortunately bike fatalities are rare, but it's false to say head injuries aren't relevant.

http://www.helmets.org/stats.htm

The data: http://www.helmets.org/stats.htm

Short version: helmets reduce incidence of serious head injuries by ~90 percent. Face injuries by ~75 percent.

Quoting:

In bicycle crashes, 2/3 of the dead and 1/8 of the injured suffered brain injuries.

95% of bicyclists killed in 2006 reportedly were not wearing helmets.

Scientific research shows that, for example, wearing a cycling helmet makes no difference. A helmet will not protect you in a serious accident and the slight increase in the risk taking behaviour you engage in by wearing one balances out the benefit you'd get from it, when compared to not wearing one when you're in a minor accident.

Actually, as I understand, scientific research shows that wearing a helmet has a tremendous benefit in prevent the sorts of injuries that leave people brain damaged for the rest of their lives. Apparently they reduce mortality rates by around 33%. The whole "you'll take more risks" thing sounds like neo-conservative pablum dolled out by idiots who care more about ratings than facts.

Here's some links to educate yourself:
http://www.helmets.org/stats.htm
http://www.bhsi.org/henderso.htm

See here.

So sure you guys wiped out multiple times when you were kids and didn't get hurt. that has nothing to do with what I wrote. What kills people and seriously injures people in bicycle accidents isn't the ground that is traveling at a relative velocity of 15MPH when the slide out on the bike, it's the vehicle T-Boning or side swiping them at 35MPH, or the hoods and tempered windshields that are a lot harder than dirt.

A higher proportion of the population live in urban environments than even 20 years ago (let alone 30 or 40 years), and those environments have more cars sharing the road with those bikes than 20 years ago, so you can expect a higher rate of injuries from bicyclists being hit by motor vehicles just because the number of possible encounter situations have increased proportionately. And if you think cutting the number of brain injuries by 45% to 88% isn't often, then I'd like to know what is. Because while bicycle accidents may only compose about 1.8% of motor vehicle accident deaths, there's also a lot fewer bikes on the road than trucks or cars. So while legally requiring bike helmet use doesn't make a big difference in the number of vehicle deaths, it does make a significant difference in the number of deaths and serious head injuries for cyclists.

Then again, maybe letting people who refuse to understand that allow their kids ride around on streets without bike helmets would help clean up the gene pool.