In 1298 Emperor Andronicus II, defeated by Serbian King Stefan Milutin, promished him an imperial bride. He intended his sister, Eudokia, empress dowger of Trebizond, but she refused and Andronicus then selected his daughter Simonis or Simonida, who was born about 1294. The 50 year old Milutin divorced his third wife, Anna Terter, sister of the Bulgarian Tsar, and they were married in 1299 despite protests from the Byzantine clergy. after Milutin died in 1321 Simonida returned to Constantinople and entered a monastery, dying sometime after 1345.
It has been claimed that Militun did not wait until Simonida was old enough, but had sex with her soon after the marriage, and damaged her reproductive organs making her unable to have children. If that story is correct, it is sort of surprising that she was never accused of poisoning or assassinating Milutin.
One account says the Patriarch of Constantinople was horrified by the idea of marrying off Simonida so young, but as unable to find any religious authority for absolute minimum age for marriage and absolute prohibition of marriage below any specific age.