I've never in many a year of riding found/heard a consistent "guideline" for helmet replacement except as you have already mentioned (after a direct impact/crash). I've heard a Bell "spokesperson" type say every three years or so, but who knows. I've never had one long enough to have it age out as it were. I've had to get one every year or so (give or take) due to crashes and such. I know extreme heat can destroy them (leaving them in the car during summer's hot temps) pretty quick.