Read this book, it is one of the most enjoyable Sci-Fi books I have read for a while. I have been reading the Scalzi's Human Division series books and they are good, but this is better.
The concept of 75 year old's enlisting (after starting the process 10 years before) to become soliders in an off-world force (Colonial Defence Force), giving up all their rights and breaking all contact with anyone left on Earth is not something you read everyday. The book is logical in terms of how a pensioner on Earth can become a superhuman soldier, the method is not really a surprise but that is an advantage.
Action is fast paced, with the effects of being a soldier consider and then moves on. John Perry is the central character, who we see going from enlistment, meeting new friends, physically becoming the superhuman solider, training, fighting a number of aliens and becoming a veteran - all in a very short period of time. Along the way we see him make and lose friends.
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