were did the saying "once a man twice a child" start?

Answer 1

it is an old saying, a proverb, but not Biblical. it is something accepted as a truth by many, and similar to sayings such as "the early bird catches the worm", it would be hard /if not impossible to track down when it actually started, you could only maybe track when it became popular and common within a certain group, by researching when it first appeared in the written word, basically one day someone said it, someone heard it and repeated it, and it was passed around.
it refers to man entering like a second childhood later in life, but has gained popularity in more recent times in reference to a book of the same name, written concerning John Lennon,

Answer 2

It is found in Shakespeare in Hamlet and King Lear