Well, it was brought in a bit earlier than that - the first episode had this exchange (quoted from memory):
THE DOCTOR: What happened? AMY: I grew up. THE DOCTOR: We'll soon fix that.
Though I'm not sure it's as easy as either refusing to grow up or growing up. The Doctor has grown up, several times over; he's a very old being, far older here than he was during RTD's reign. He is wise enough to know what he can do; and that perhaps, running from (or rather not acting on) that responsibility - yet not forgetting it - can sometimes be best for everyone. This is the continuation of the Doctor who pronounced himself "A Timelord Victorious", after all.
no subject
THE DOCTOR: What happened?
AMY: I grew up.
THE DOCTOR: We'll soon fix that.
Though I'm not sure it's as easy as either refusing to grow up or growing up. The Doctor has grown up, several times over; he's a very old being, far older here than he was during RTD's reign. He is wise enough to know what he can do; and that perhaps, running from (or rather not acting on) that responsibility - yet not forgetting it - can sometimes be best for everyone. This is the continuation of the Doctor who pronounced himself "A Timelord Victorious", after all.