1) when I crash,
2) when something new and cool comes out,
3) when the existing helmet gets disgusting.
As for the 3 year bit, a helmet will lose its ability to absorb energy over time as it breaks down, and as it absorbs minor impacts, being dropped, thrown in trunks, etc.
But no one has good data on how much capability it loses, or how fast, so the manufacturers to cover ass, and sell helmets put out stuff, like replace your helmet every 3 years.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.