I think the present birthdate of 1057 is unlikely, given Erland's participation in Harald Sigurdsson's ill-fated invasion of the Kingdom of England in 1066. His mother, Ingibiorg, was the niece of Harald Hadrada of Stanmore Bridge fame, and her sons went with Harald to Byzantium and were with him in the Varangian Guard. They then accompanied him on the campaign of 1066 - being left in charge of the baggage camp on the day of Stanford Bridge.
So, in my opinion, it is unlikely that Erland was 9 in 1066.
Given his parents' marriage is likely to have been about 1044/6, I'd estimate his birthdate at about 1050 - making him 16 in 1066, and his older brother, Paul's as 1048 - making him 18 at Stanford Bridge.