I do believe that it depends on how often it’s used and how much sweating you do in it. During Jun/Jul I sometimes wash the liner after just a few rides, (got 2 sets) I'm not keeping that helmet for 5 years. I've ridden only 21,000 so far this year and about 29,000 in that helmet (a Shoei Neotec 3).
...
While miles worn does sorta indirectly have some impact on a helmets 'safely usable' life; and how much sweat they absorb does have a fairly big impact on the life of the pads & liners, so it is a great idea to at least clean those pads & liners, if not replace them more often; it's the
light and
ambient temps our helmets are exposed to that have the greatest impact! Just exposing your helmet to daylight starts degrading the integrity of the shell, while exposure to ambient temps plus any added heat from your head starts that and the rigid padding stuff inside deteriorating over time. So unless you keep your helmet in a cooler/the fridge whenever you're not riding, then wear an insulating cap
all the time you are riding/have your helmet on,
and you
never ride during daylight hours nor let your helmet be exposed to anything that mimics sunlight in any way (so basically, never let it be exposed to any light at all!) then it pretty much starts degrading once it comes outta the box and experiences the ambient temps/sees the light of day! And there's
NOTHING you can do to stop that degradation once it's started; altho exposure to even more sweat, heat, &/or light can and does speed it up!
And yeah, some helmets that maybe see less use, sweat, heat, &/or light than others used daily
might degrade a little slower, so they could possibly/maybe last a little bit longer than the 5 or so years that's generally recommended as their safe life and still be able to do their job, but after seeing
just one helmet that was only about 7 years old which had degraded to the extent that the outer shell lost its integrity and splintered in multiple places while the interior rigid padding turned to powder everywhere it was pressured under what was really a relatively light tap on the road surface & kerbing (compared to the hit it
could have been - he was doing 60 mph just moments before!) from a relatively slow speed spill and slide into the gutter at about 20 mph, I'm happy to stick to that 5 year recommendation and
KNOW that there's likely to be a little bit more life/safety margin in my helmets 'safely usable' life, especially since I get mine tested regularly.
But really, whatever reasoning you use to justify it to yourself, swapping out your 5+ year old helmet for a new helmet and then keeping it in good condition, clean, & bump free, is a
GREAT idea!
) and while I was learning to do everything again, I saw lots of others going thru similar issues, and I learnt how debilitating even just a few mild concussions can be! 
and I can’t find the same decal or stickers to apply to new. Minor complication’s & then there’s that bud who has like 6 or 8 different helmets & barely rides - his only vehicle.
