Replace your Helmet?

I have two helmets that I have owned for years. My Bell Moto 3 is so old that the fabric has pulled loose from the edges and the glue is deteriating. This was a prime helmet in it's day many years ago for the dirt. How long ago did I buy this helmet? Well, lets just say it was a few years back. So far back that when I tried it on for a hoot it barely fits my head now and gave me a crushing headache after only a couple of minutes. I guess my stomach isn't the only thing that has gained girth after all these years.
My other old helmet is a Shoei that is 23 years old. It still fits fine but it is past it's prime in safety for the street. You should replace your helmet every five years; if it has had any substantial impact it should never be worn again. A helmet is designed to crush both the shell and the inside foam to absorb the impact to the head. Once a helmet has been in a crash it should not be worn again even if you can't see the damage. There may be only some scaring on the outer shell but the helmet is weakened and if you should have another crash it will fail.
Take a windshield on a car for example; once it gets a crack it is weakened. You can still use it but should it be hit again it will crumble easier.
Want an idea of how a helmet protects your head? Take an egg and wrap it in foam and let it fall off the counter top. The foam helps absorb and distribute the impact; drop an un-wrapped egg off the counter and see what happens.
Another reason to replace your helmet every five years is because the resins and glues begin to breakdown. Petroleum based cleaners can also break down the outer shell of a helmet. Solvents in paints may also degrade the helmets shell. Makeup, hairspray and just plain old wear and tear contribute to the degradation of a helmets materials.

All helmets are not alike. There are SA, K and M designated helmets.
SA standard helmets are used in auto racing and have flame retardency while an M for motorcycles does not.
An SA helmet also has a narrower field of vision and some of them are not street legal.
SA standard helmets are also rollbar impact tested while M helmets are not.
An K standard helmet meets similar SA standards with the exception that they have no flame retardency.
M standard helmets meet rigid testing but are not flame retardant, and the field of vision is wider than an SA rated helmet. This is especially true in a full face helmet. There are also several novelty helmets out there such as shorty helmets that are not even D.O.T. approved.

I had another old helmet that is probably still at my parents house down in the basement somewhere. I took a spill with it on and several small cracks appeared at the crown of it. Over time those small cracks became large cracks so I quit using it.

And one last thing to note. That chin strap on the helmet; it's meant to be used to secure the helmet to your noggin. What good is wearing a helmet that's just going to fly off in an accident? Helmets aren't cheap and we are reluctant to shelve them after a few loyal years. If you still want to wear the old helmet you can contact the manufacturer for a recommendation and perhaps an inspection. Better to be out a few bucks than outta time.