Traditionally, before the age of birth certificates, majority was based less on age and more on whether the individuals concerned exhibited culturally recognised signs of puberty (such as menstruation for females). Even the traditions which defined majority by age did so at a much lower age (the Jews celebrate bar-mitzvah at 13, for instance). How did 18 which, as far as I know, does not conform to any biological process become the universally recognised age of puberty?