John, the eldest son of King Manuel I to his second wife  Maria of Aragon, was born in Lisbon on 7 June 1502.  The event was marked by the presentation of Gil Vicente's Visitation Play or the Monologue of the Cowherd  in the queen's chamber. The young prince was sworn heir to the throne in 1503, the year his youngest sister, Isabella of Portugal, Empress Consort of the Holy Roman Empire between 1527 and 1538, was born. John was educated by notable scholars of the time, including the astrologer Tomás de Torres, Diogo de Ortiz, Bishop of Viseu, and Luís Teixeira Lobo, one of the first Portuguese Renaissance humanists, rector of the University of Siena  and Professor of Law at Ferrara . John's chronicler António de Castilho said that, "Dom João III faced problems easily, complementing his lack of culture with a practice formation that he always showed during his reign" . In 1514, he was given his own house, and a few years later began to help his father in administrative duties. At the age of sixteen, John was chosen to marry his first cousin, the 20-year-old Eleanor of Austria, eldest daughter of Philip the Handsome of Austria-Burgundy and Queen Joanna of Castile, but instead she married his widowed father Manuel. John took deep offence at this: his chroniclers say he became melancholic and was never quite the same. Some historians also claim this was one of the main reasons that John later became fervently religious, giving him name the Pious .
Answer this question: How many years old was JOhn when he was sworn heir to the throne?
1