Puberty

Puberty was, according to the Eleventh Doctor, "always a funny time." (TV: Night Terrors) As Gwen Cooper claimed, during this time, one was "not quite adult, not quite child." (TV: Children of Earth: Day Four)

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.