score:44
I'd imagine that the youngest ever bishop would have to be Prince Frederick, Duke of York and Albany, the second son of King George III.
Born on 16 August 1763, he was appointed as Prince Bishop of Osnabrück on 27 February 1764, at the age of just 6 months and 11 days!
He would be the last Prince Bishop of Osnabrück.
An interesting side-note is that James Burgh dedicated his 1766 volume of collected essays on religious toleration, contemporary politics, and educational theories, Crito: Or, Essays on Various Subjects, to:
The Right Rev. Father
(Of three years old)
His R.H FREDERIC [sic]
Bishop of O.
Upvote:2
Although I'm not breaking the records set by other answers, I would add the example of Cesare Borgia who was made bishop while being younger than James of Nicosia, as Cesare Borgia was made bishop of Pamplona at 15 years old.
Additionally, he was made archbishop (of Valencia) at 17 and cardinal at 18.
Upvote:5
Perhaps: Theophylactus of Tusculum was elected Pope Benedict IX (aka "Bishop of Rome") in the year 1032.
Wikipedia says this about his age:
Horace K. Mann, writing in the Catholic Encyclopedia says Benedict IX was about 20 when made pontiff in October 1032.[3] Other sources state 11 or 12,[4] based upon the unsubstantiated testimony of Rupert Glaber, a monk of St. Germanus at Auxerre.[5]
Upvote:5
Luis Antonio Jaime of Spain (25 July 1727 – 7 August 1785), Infante of Spain, Cardinal Deacon of the titular church of Santa Maria della Scala in Rome, Archbishop of Toledo and Primate of Spain, 13th Count of Chinchón, Grandee of Spain First Class, known as the Cardinal Infante, was a son of Philip V, King of Spain and his second wife, Elisabeth Farnese.
He was 8 years old when he was made both a Bishop and a Cardinal.
He is listed in the Guinness Book of World Records as the youngest-ever cardinal.
Upvote:8
In 933 Emperor Romanus I Lekapenos appointed his youngest son, Theophylaktos (917-956) Patriarch (and thus bishop) of Constantinople aged 16.
Rannuccio Farnese (1530-1565) was made a Cardinal-deacon in 1545 age 15 by his grandfather Pope Paul III. He was also granted several bishoprics, including titular Latin Patriarch of Constantinople in 1546 age 15 or 16. His oldest brother Alessandro (1520-1589) was appointed Bishop of Monreal in Sicily in 1536 aged 15 years, 7 months, and 10 days.
I am sure I have heard of younger bishops than those, but I can't seem to find any mentioned online at the moment.
I know that there have been a few cardinals in the Renaissance who were younger than 16, but they could have been cardinal-deacons or cardinal-priests instead of cardinal-bishops.
Upvote:15
Hugh of Vermandois became Archbishop of Reims in 925, at only five years old. It was part of a rebellion by his father, Hugh II of Vermandois. Hugh II joined with Robert, Count of Paris, and father of Hugh Capet, in a rebellion against their king, Rudolph of France. They were supported by Otto I. When the rebellion failed in 931, the previous Archbishop was reinstated.
The Archbishop of Reims was one of the most influential seats. It crowned the King of the Franks, and would soon ally with the Capets.