Just wondering if anyone has read Old Man's War by John Scallzi. The book was originally published in 2005 but is worth considering if you have not read it. The book contains some very interesting idea's on the future of humanity. The hero of the story joins the army at the age of 75 and is given a new enhanced body to become a more effective soldier. In the future it seems the army wants experienced people with knowledge and skills.
As well as the excellent military elements there's also some moral aspects to the story to get you thinking. It is one of the few physical books I have managed to keep and will re-read it in the future.