[gloob]

Which is the cutoff?

Pick one arbitrarily. It's inelegant, sure, but we use it for all sorts of other things. Significant example: are you an adult? All sorts of perfectly intelligent, rational arguments could be raised saying "It depends on the person, now doesn't it?" The law, however, says: If you have been alive for X number of years, you are an adult. Otherwise, you are not. It seems to work halfway adequately for that.