Around 762 Æthelred I of Northumbria was born to Æthelwald Moll King of Northumbria.
In 774 Æthelred I of Northumbria (age 12) was appointed King Northumbria.
Anglo-Saxon Chronicle. 774. This year the Northumbrians banished their king, Alred, from York at Easter-tide; and chose Ethelred (age 12), the son of [his father] Mull, for their lord, who reigned four winters. This year also appeared in the heavens a red crucifix, after sunset; the Mercians and the men of Kent fought at Otford; and wonderful serpents were seen in the land of the South-Saxons.
Anglo-Saxon Chronicle. and Osred, who had been king of the Northumbrians, returning home after his exile, was apprehended and slain, on the eighteenth day before the calends of October. His body is deposited at Tinemouth. Ethelred (age 32) this year, on the third day before the calends of October, took unto himself a new wife, whose name was [his wife] Elfleda.
Anglo-Saxon Chronicle. Ethelred (age 34), king of the Northumbrians, was slain by his own people, on the thirteenth day before the calends of May; in consequence of which, Bishops Ceolwulf and Eadbald retired from the land. Everth took to the government of Mercia, and died the same year. Eadbert, whose other name was Pryn, obtained the kingdom of Kent; and Alderman Ethelherd died on the calends of August. In the meantime, the heathen armies spread devastation among the Northumbrians, and plundered the monastery of King Everth at the mouth of the Wear. There, however, some of their leaders were slain; and some of their ships also were shattered to pieces by the violence of the weather; many of the crew were drowned; and some, who escaped alive to the shore, were soon dispatched at the mouth of the river.
On 18 Apr 796 Æthelred I of Northumbria (age 34) died.