<P> The first narrative account of Arthur 's life is found in Geoffrey of Monmouth 's Latin work Historia Regum Britanniae ( History of the Kings of Britain ) , completed c. 1138 . This work is an imaginative and fanciful account of British kings from the legendary Trojan exile Brutus to the 7th - century Welsh king Cadwallader . Geoffrey places Arthur in the same post-Roman period as do Historia Brittonum and Annales Cambriae . He incorporates Arthur 's father , Uther Pendragon , his magician advisor Merlin , and the story of Arthur 's conception , in which Uther , disguised as his enemy Gorlois by Merlin 's magic , sleeps with Gorlois 's wife Igerna at Tintagel , and she conceives Arthur . On Uther 's death , the fifteen - year - old Arthur succeeds him as King of Britain and fights a series of battles , similar to those in the Historia Brittonum , culminating in the Battle of Bath . He then defeats the Picts and Scots before creating an Arthurian empire through his conquests of Ireland , Iceland and the Orkney Islands . After twelve years of peace , Arthur sets out to expand his empire once more , taking control of Norway , Denmark and Gaul . Gaul is still held by the Roman Empire when it is conquered , and Arthur 's victory naturally leads to a further confrontation between his empire and Rome 's . Arthur and his warriors , including Kaius ( Kay ) , Beduerus ( Bedivere ) and Gualguanus ( Gawain ) , defeat the Roman emperor Lucius Tiberius in Gaul but , as he prepares to march on Rome , Arthur hears that his nephew Modredus ( Mordred ) -- whom he had left in charge of Britain -- has married his wife Guenhuuara ( Guinevere ) and seized the throne . Arthur returns to Britain and defeats and kills Modredus on the river Camblam in Cornwall , but he is mortally wounded . He hands the crown to his kinsman Constantine and is taken to the isle of Avalon to be healed of his wounds , never to be seen again . </P>
<P> The first narrative account of Arthur 's life is found in Geoffrey of Monmouth 's Latin work Historia Regum Britanniae ( History of the Kings of Britain ) , completed c. 1138 . This work is an imaginative and fanciful account of British kings from the legendary Trojan exile Brutus to the 7th - century Welsh king Cadwallader . Geoffrey places Arthur in the same post-Roman period as do Historia Brittonum and Annales Cambriae . He incorporates Arthur 's father , Uther Pendragon , his magician advisor Merlin , and the story of Arthur 's conception , in which Uther , disguised as his enemy Gorlois by Merlin 's magic , sleeps with Gorlois 's wife Igerna at Tintagel , and she conceives Arthur . On Uther 's death , the fifteen - year - old Arthur succeeds him as King of Britain and fights a series of battles , similar to those in the Historia Brittonum , culminating in the Battle of Bath . He then defeats the Picts and Scots before creating an Arthurian empire through his conquests of Ireland , Iceland and the Orkney Islands . After twelve years of peace , Arthur sets out to expand his empire once more , taking control of Norway , Denmark and Gaul . Gaul is still held by the Roman Empire when it is conquered , and Arthur 's victory naturally leads to a further confrontation between his empire and Rome 's . Arthur and his warriors , including Kaius ( Kay ) , Beduerus ( Bedivere ) and Gualguanus ( Gawain ) , defeat the Roman emperor Lucius Tiberius in Gaul but , as he prepares to march on Rome , Arthur hears that his nephew Modredus ( Mordred ) -- whom he had left in charge of Britain -- has married his wife Guenhuuara ( Guinevere ) and seized the throne . Arthur returns to Britain and defeats and kills Modredus on the river Camblam in Cornwall , but he is mortally wounded . He hands the crown to his kinsman Constantine and is taken to the isle of Avalon to be healed of his wounds , never to be seen again . </P>
how old was arthur pendragon when he became king
how old was arthur pendragon when he became king
fifteen
fifteen