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)

Knowing that the 456 collected pre-pubescent children, Gwen suggested Clement McDonald was on the cusp of puberty in 1965, hence why they left him behind. (TV: Children of Earth: Day Four)

As observed by the Twelfth Doctor, Grant's puberty caused a reaction in the Hazandra gemstone in his body, causing him to see everyone naked through X-ray vision. (TV: The Return of Doctor Mysterio)

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.