1165. He based his argument on a document from 26 April 1177, in which Leszek was mentioned as taking the last place among Polish princes, after Mieszko "junior" (Misico iunior dux). Balzer considered that this meant that Leszek was younger than Mieszko, who according to Balzer was Mieszko III the Old's son Mieszko the Younger, who was born after 1159. Balzer also thought that Leszek had to be at least 12 years old in order to be considered an official witness . This date proposed by Balzer had thus been accepted in historiography. However, his argument was refuted as the Mieszko "junior" mentioned in said document was shown instead to be Mieszko IV Tanglefoot (born before 1147), and also that a prince could be mentioned in document as a witness even when he was less than 12 years old. Based on the tympanum from Ołbin made in 1172, historian Kazimierz Jasiński supposed that at that time Leszek was around 10 years old, and therefore he would have been born around 1162. These idea is accepted by some historians. Borys Paszkiewicz considered that the year of birth given by Długosz (1158) could be right. Duke of Masovia and Kuyavia The death of his older brother Bolesław in 1172 left Leszek as the only heir of his father. High Duke Bolesław IV, reportedly devastated by his first-born son's death, died one year later (5 January 1173). He left Masovia (and Kuyavia, which was in the 12th century a part of Masovia) to Leszek, at the age of eleven or less. The overlordship of Poland, which included the control over Kraków and Gniezno, was taken by the eldest surviving brother of Bolesław IV, Mieszko III the Old. According to his father's will, Leszek began his rule under the guardianship of his youngest uncle, Casimir II the Just. When in 1177 Casimir II became prince of Kraków, he nominated the magnate Żyron as a guard for Leszek, who suffered from extremely poor health. According to the 18th-century Russian historian Vasily Tatishchev, based on older historiography, Leszek fought against Prince Volodar of Minsk, who in 1180 captured Brest, which belonged to his brother-in-law, Vasilko Iaropolkovich. After a long and exhausting war, Brest was ultimately not recovered, and Vasilko gave all the rights over this land to Leszek. This account is now considered as unreliable, however. At some time before 1186 Leszek unexpectedly changed his dynastic