@Invictus, I figured it was so self-evident that it didn't even need discussion.
OK - here I go:
PRO: The elderly are more disposable... we are actually all disposable, but they are more so. They are less likely to have minor children to support, less likely to have living parents that would grieve their deaths and besides, they have all that experience to draw upon to make them real fighting machines. With the advents of technology, after all, its not like they'll be expected to physically fight... well, except each other in the local bar when on leave, of course. They'll be pushing buttons like the rest of us... and when their deaths are recorded by the master computer system, they can dutifully report to the local disintegration chamber. (Star Trek reference)... It is, after all, such things that hold our society together. Without the orderly death of those selected for termination, chaos and the destruction of our entire civilization would surely ensue. Further, far more CEOs, government officials and other leaders are elderly than are 18-25 years old. Making the elderly fight our wars would make it more likely that policy makers would hesitate before declaring war and they themselves could be drafted. Oh - and they smell bad.
Con: Old people are stupid. ...and ugly. Making pre-teens fight our wars would make much more sense. They don't have wrinkles, they have boundless energy, aren't yet interested in gay sex (so "Don't Ask Don't Tell" won't even be an issue), they like playing war anyway, and there isn't as much investment of time made in them... we can always pop out a replacement should one break or die or something. Besides... since their smaller, wouldn't that make our wars smaller? That would be an immediate improvement. Besides, maybe war would be cute. That would be nice. You know those cute photos of little four year old kids in big person clothes holding hands and kissing? We could raise money for wars by selling cute photos of four year olds in baggy uniforms like running each other through with bayonets.
Real: Why can't we all just get along? (no, really... I mean it).