I agree it's archaic, which is why I'm surprised it's still being cited as a universal moral principle in a way that, say, the obligations of blood feud are not. After all, as you imply, the two things belong to the same matrix of values and duties. I believe you're right by the way that men past arms-bearing age would have been considered illegitimate targets then - but are they now? Going by the "women and children" formula, one would think not.

Having said it's archaic to believe that all men, and only men, are potential soldiers, though, it's still unfortunately the case that with the exception of Israel every country that has compulsory military service applies it to men only - including such supposedly feminist-friendly states as Sweden, Denmark, Norway and yes, even my beloved Finland. As to why there haven't been more marches by women in those countries demanding the right to be conscripted, one can only speculate!