One example that springs to mind right away is the king of Sicily and future Holy Roman Emperor Frederick II, who was 14 years old in 1209 when he married Constance of Aragon, whose was probably about twice his age.

So how did this happen? Well first we have to go back to 1154, when king Roger II of Sicily died. He was succeeded by his eldest surviving son, William I. However, Roger II also had other sons who died before him, including one also named Roger. This younger Roger (who had died in 1148) had an illegitimate son named Tancred. Roger II also left behind a pregnant wife, and his posthumous daughter, Constance, was born later in 1154. William I ruled Sicily until his death in 1166, when he was succeeded by his son William II, who ruled until 1189.

William II had no children, so Constance was his closest legitimate heir. She had married the Holy Roman Emperor, Henry VI, a few years earlier in 1186. But the nobles of Sicily, and the Pope, were worried about the imbalance of power if the Emperor also inherited the Kingdom of Sicily through Constance. In 1189, the kingdom was seized by her bastard nephew Tancred, until he died in 1194. Tancred’s son, William III, was still a child, so Constance easily overthrew him and took back the kingdom, which, as far as she was concerned, was rightfully hers.

Constance was pregnant at the time and in December 1194 she gave birth to Frederick. When Frederick was still only 2 years old in 1197, his father emperor Henry died, but once again neither the Sicilian nor German nobles, nor the Pope, wanted one person to control both the Empire and Sicily. They forced Constance to give up Frederick’s claim to the Empire. Unfortunately Constance also died the next year in 1198, leaving Frederick an orphan. The church felt it was the special protector of orphans, and this orphan could potentially be the most powerful ruler in Europe, so the new pope, Innocent III, promised to be Frederick’s foster father and took the entire kingdom of Sicily under his protection as a “papal fief.” Of course, Innocent would be affected the most if Frederick inherited both Sicily, in southern Italy, and the Empire, which included much of northern Italy - the Papal States, in the middle, would then be surrounded. Innocent wanted to prevent that outcome.

The Empire ended up in a civil war for almost Innocent’s entire papacy, which suited him just fine. Meanwhile he made sure Frederick was educated and fit to rule Sicily, and this included arranging a marriage for him. Marrying a German princess would not work - that would just end up in the same situation as Constance and Henry. Instead, Innocent made an alliance with the king of Aragon, Pedro II, who sent his sister - also named Constance.

Constance of Aragon’s date of birth is not known exactly but she was probably born around 1184, as she married king Emeric of Hungary around 1199 when she was about 14 or 15. She would have been too young to marry if she was born after that, but she could have been born a few years earlier, and may have been about 20 when she married Emeric. Emeric died in 1204, their infant son Laszlo died in 1205, and Emeric’s brother Andrew became king of Hungary. Constance returned to Aragon and a few years later Pedro II and Innocent III arranged a second marriage for her, with Frederick.

They married in August 1209 when Frederick was still 14 and she was probably at least 25 or maybe 30 or so. In 1211 they had a son, named Henry after Frederick’s father. He was their only child.

Meanwhile, the civil war in Germany had ended up with two rival claimants. One, Philip of Swabia, was assassinated in 1208 by the other claimant, Otto of Brunswick. But Otto was unpopular and the German nobles who elected the king of Germany wanted to choose Frederick instead, which they did in 1212 when Frederick turned 18. Otto was overthrown in 1215, but it took another few years before Frederick could be properly elected king of Germany in 1220, and then ultimately crowned as Holy Roman Emperor by the new pope, Honorius III (Frederick’s foster father Innocent III died in 1216).

Constance of Aragon was now Frederick’s empress, but she died not long afterwards in 1221. In 1225 Frederick married again, but this time the situation was reversed. He was now about 30, and his new wife, queen Isabella II of Jerusalem, was only 13. Isabella died in childbirth only 2 years later at the age of 15. Their son, Conradin, ended up claiming Sicily, the Empire, and Jerusalem, which caused all sorts of problems and warfare all across the Mediterranean, but that is a different story…

So, this is perhaps not an enormous age gap, and it wasn’t exactly a forced marriage. Frederick was 14 years old, almost 15, which was the age of majority in medieval Sicily, and also beyond the age of majority for marriage according to the church (which was 13). He could participate in decisions affecting him. But the marriage was arranged by Innocent III and Pedro II of Aragon, and Constance was at least 10 and maybe 15 years older than Frederick.

Sources:

David Abulafia, Frederick II: A Medieval Emperor (Oxford University Press, 1992)

Thomas C. Van Cleve, The Emperor Frederick II of Hohenstaufen: Immutator Mundi (Oxford University Press, 1972)