Answer
No, Manuel II Palaiologos was not the eldest Byzantine Emperor at his accession. He was the youngest of the four sons of Emperor John VIII Palaiologos and his wife, Helena Dragaš. When his father died in 1448, Manuel's brother, Constantine XI Palaiologos, became emperor. However, Constantine was captured by the Ottoman Turks at the Battle of Varna in 1444 and died in captivity in 1453.