I mostly enjoyed the book. It has a lot of semi-hard science fiction in it ("you don't have the math for it" is a recurring line) and he gives his characters character, even the aliens, of which there is a formidable selection. The hook is that earth is locked out of space except for colonists (who are mostly from Third World countries) and the colonial defense forces, which one has to be 75 years old to join. The draw for the old people (they take men and women equally, contrary to the title's suggestion) is they will be rejuvenated by the CDF, which has vastly superior technology to earth's.
The book reads like a mid-career novel. It's not quite a page-turner, but close enough. There are a couple of points where characters enter or leave the book more as contrived plot points than anything else, and Scalzi over-uses the "monologue as exposition" ploy just a tad bit. That's the trick of having a character spell out some important background information in the form of a lecture or one-sided chat rather than invoking a narrator. OTOH when you tell a story in the first person, that's something you get stuck doing, like it or not.
The love interest is somewhat predictable, it's telegraphed a few times early in the book. There are some wry moments of humor when the protagonist runs through the setup wizard on his new CDF-issued PDA which made me chuckle, but there isn't a lot of humor in the book. It could use more. Thankfully, there was also no bacon, no cat, no tape.