Puberty

Puberty was a time when, as Gwen Cooper claimed, one was "not quite adult, not quite child." (TV: Children of Earth: Day Four) According to the Eleventh Doctor, puberty was "always a funny time." For this reason, he suggested he would check up on George, a Tenza disguised as a human child. (TV: Night Terrors)

Behind the scenes
Steven Moffat joked that the Doctor had "hit puberty" (DOC: The Doctors Revisited - The Eighth Doctor) when the Eighth Doctor shared their first on-screen kiss, with Grace Holloway in the 1996 tele-film.