Nurture over nature

Who said "Give me your child until he is 9 and he will be mine for life".

I googled your phrase in all sorts of combinations and found many different sources:

communism
Jesuits
some religion
"a wise pope"
a bishop

In other words, I found nothing except a whole bunch of maybes.

The behaviorist psychologist John B. Watson said something like that:

"Give me a dozen healthy infants, well formed, and my own specified world to bring them up in and I'll guarantee to take any one at random and train him to become any type of specialist I might select--doctor, lawyer, artist, merchant, chief, and yes, even beggar-man and thief, regardless of his talents, penchants, tendencies, abilities, vocations, and race of his ancestors. There is no such thing as an inheritance of capacity, talent, temperament, mental constitution, and behavioral characteristics" ["Behaviorism," New York: Norton, 1925].