So true - I remember a book for children - it may even have been one of the Pooh stories, in which reference was made to quite old people, some even over 20! Don't forget, one of Milne's books was 'Now we are seven' aimed at that age group, of whom I would have been one at the time, and to whom 20 would have seemed incredibly old - although not as old as my parents, respectively 41 and 32 at the time - ah, sweet mysteries of youth!
Mike