Europe is now more or less consistently 18 for parental consent, but marriages at a younger age are still permitted across the EU with judicial consent.
I mean, are there any children who are citizens of the Vatican City, given its bizarre citizenship situation? (Citizenship is based, essentially, on _employment_, and lapses when employees retire; sole exception, AIUI, is retiring popes, who get a free pass.) I would suspect that this is a holdover from the Papal States (which bit the dust in 1870, when this sort of thing was more acceptable), which someone finally got around to cleaning up.
I suspect that this norm lingers from times of stricter social norms and before the pill, when a marriage would be a way to avoid a scandal when the prospective bride were pregnant.