My definition of society is the rules and conventions by which we operate.
It's hugely complicated and ultimately subjective (which I believe I provided for in my post) and I'll try not to sound too pompous, but in my view a civilised society is one that balances the rights of the innocent and the guilty in a way that is proportionate, and one that also recognise that, to some extent, we can all fall into both categories and that some people are capable of change - and the younger they are, the more capable they are of so doing.
More specifically to the point in hand, I believe that closing the door on someone who has faltered in their youth (and I would count up to 21 in this regard (although on a downward sliding scale from the age of 18 onwards, in particular)) is not what I would consider civilised, even if they have committed terrible acts before they are legally adults. I felt the same way about Venables and Thompson - one of whom appears to have rehabilitated himself, the discernibly other less so.
My view about people lacking compassion was probably not expressed in a way that was nuanced enough, and Begum possibly descends into a grey area (which I should have provided for in my post) but I do believe that anyone who wholly fails to concur with the point I make about drawing a distinction between a 16 year old and (say) a 22 year old palpably lacks sufficient compassion imo, and if they do agree there is a distinction to be drawn, then I fail to see how my view on Begum is inconsistent with that.
I accept the last part of your post may, to some extent, render many people's views replete with inconsistency, myself included, and all I can do is try and find a position that falls most squarely within my view on the subject in order not to be hypocritical around it, through the prism of Begum.
I feel that someone who makes a mistake (even to the extent of Begum) from the ages of fifteen to eighteen (when her principal 'offending' appears to have occurred) should be, at some point, afforded the opportunity to contribute to society again, once they've been punished for that offending, and for her, in my view (as previously posted) that should be when she's in her forties, assuming she's sufficiently demonstrated genuine remorse and regret for her actions.
I hope that deals with you enquiry sufficiently.