I've removed Ælflæd 's birth date altogether as I see no sources to support even guesses.
Osbeorn is portrayed as a young boy in Shakespeare's Macbeth - and part of his theme of sons of fathers that showed up Macbeth's own childlessness. Both Banquo and Macduff's sons as young boys feature prominently in the play. So ten years old is just barely feasible in these terms; but there is no good reason to imagine that Shakespeare was using anything but creative licence here - so I've removed.
Thanks for the headsup