@gmh but that's the point, no? Absent the existence of external reference points, nevertheless some points must be defined as the border between one thing and the other, and the idiom makes the underlying arbitrariness of the specific choice quite clear.

Age of consent / maturity is an obvious example, in each of its specifics. Ten is too early, thirty perhaps a bit late, but e.g. for drinking beer, Germany says 16, the UK 18, the USA 21. Lines in the sand, nevertheless legally quite strict.