by [email protected] » Fri Mar 22, 2013 8:04 am
Okay this is not my original theory but it makes sense!
1) The Face of Boe is only a head.
2) The Headless Monks cut off their heads, but they heads continue living in a separate box
3) We know that Jack is aging, even if it is very slowly.
Therefore...
Jack has a run in with the headless Monks, they chop his head off and after that, no one kills Jack meaning the rest of his body never grows back. So Jack lives out the rest of his life as the Face of Boe.
Also, Moffat originally planned to have Jack in "A Good Man Goes to War", but Barrowman couldn't make it because of a scheduling conflict with "Torchwood: Miracle Day".