Æthelred was the son of Penda of Mercia. Penda's queen, Cynewise, is named by Bede, who does not mention her children; no other wives of Penda are known and so it is likely but not certain that she was Æthelred's mother. The "Anglo-Saxon Chronicle" gives Penda's age as fifty in 626, and credits him with a thirty-year reign, but this would put Penda at eighty years old at the time of his death, which is generally thought unlikely as two of his sons (Wulfhere and Æthelred) were young when he was killed. At least as likely is that Penda was fifty years old at his death, rather than at his accession. Æthelred's date of birth is unknown, but Bede describes Wulfhere as a youth at the time of his accession in 658, so it is likely he and Æthelred were in their middle teens at that time. The early sources do not say whether Æthelred was older or younger than Wulfhere.