Helmets - when to replace?

When a new more stlyish/lighter/better ventilated helmet is available

I thought it was generally around 5 years, assuming at least regular use in all conditions, because UV damage to the polystyrene and plastic degrades it over time and makes it more likely to break as it becomes more brittle.

Which feels about right,my first helmet the straps went first, tried fixing them and washing them just destroyed them.

I then ran two helmets at once,one was more commute style than full on roadie style and they both lasted for longer as were sharing the load, the commute is still going,but the roadie despite never having been dropped,bashed or hit developed a crack across the middle which was just going to keep growing.

So I'm now on my 4th in about 15-20ish years, this one might last another 2-3years,but it gets used far more than any of the others I've had due to me ramping up the miles and at more risk from off road excursions, havent mastered the duck theres an overhanging bush ahead approach yet. So might not survive that long.

Maybe when you can't get the smell out any more...

There's little evidence that helmets deteriorate with age: https://helmets.org/replace.htm

There are a few caveats: the main study of "used" helmets reported in that link took a core sample of the foam to test, and (IMHO) a core sample might miss structural weaknesses that affect the helmet's performace as a whole. To give an extreme example, if you cleanly slice a helmet in half, a core sample will presumably look as good as new, even though the helmet is in two pieces!

It's hard to control for day-to-day use. Even if you've never knowingly dropped it, could it have been bashed or otherwise impacted without your knowledge?

Technology continues to improve - even if your helmet is as good as the day you bought it, might a new one be better? I would cite MIPS as a recent technology that might make helmets safer (although Helmets.org take a skeptical view of that claim...) 

Finally, if your old helmet looks tatty or smells a bit funky, that might be as good a reason as any to replace! 

https://road.cc/content/feature/when-should-you-replace-your-cycling-hel...

My last helmet lasted 7 years. The foam pads started to deteriorate, so I replaced the helmet instead of trying to find replacement pads.

Five, maybe six years for me. I found some actual research some years back (a guy had people send him old helmets and he tested them) and unless it's been treated like crap the "every three years" recommendation by the manufacturers is either a money grab or hedging against lawsuits, depending on how generous you feel.