Earth has reached the stars and been slammed back into isolation. Humans are out there colonizing the galaxy, but Earth itself is cut off from it, becoming almost a backwater. The Colonial Defense Force (CDF) ensures that this remains so. John Perry decides to join the army and fight aliens. He gets a new, grown body (green and all) that will make him young again (even if he's not completely human anymore) and the extreme possibility of dying out in the mean universe.
He's intelligent and he rises through the ranks fairly quickly by using his brain. The friendships that Perry forms when he first enlists seem very logical, as these people have been thrown together into a strange situation with no visible support apparatus. Even the fact that the first thing these older people do when they get young bodies is to enjoy themselves with as many people as possible is certainly understandable.
The small problem with Old Man's War is that it takes a long time to get through the setup of the setting. It takes almost half the book before Perry actually gets into the action. The rest is his journey to the CDF and the establishing of the galaxy and his place in it. It is only a small problem because Scalzi does make it interesting, but I wish some of it could have been condensed.
This report prepared by eyal