at what point do you consider purchasing a new lid? if the old one ain't broke, would still consider it?

There's been some debate over this subject. I've read that you should replace your helmet every three years due to breakdown in the material from sweat, sunlight and age. I've also heard people say that three years is a bit too aggresive and one can stretch it out to five or even seven years. I personally have never had a helmet last that long. 18 months seems to be the average lifespan of my helmets.

I generally find that 18 months to two years, is about the limit for my helmets, or even before. Can't give any reason for changing them, but damaged straps is one of the causes. Not only the straps, but have they started to pull into the carcase of the helmet? All the little knocks the helmet gets in the bike shed, or out on the trail, does mean that I will not trust a helmet that has cracks or scores in the outer covering. The sweat pads that have compressed by use will cause the helmet to become uncomfortable, so once again it is due for a change. I will also change a helmet that has had a hard knock, even if there does not appear to be any further damage other than the mark of where the impact was.