Complex sentence: The adolescent (No.17) standing by the king is the royal heir Prince John (future John II of Portugal), who was born in 1455 and would have been ten to twelve years of age in 1465-67.
Split into simpler sentences: The adolescent (No.17) standing by the king is that royal heir, Prince John (future John II of Portugal). Born in 1455, John would have been ten to twelve years of age in 1465-67.