"John Perry did two things on his 75th birthday. First he visited his wife's grave. Then he joined the army."
That is the opening lines to this book. I ran across John Scalzi a few years ago when I read Redshirts. The other book I kept hearing about from him was Old Man's War. I don't think we got an exact year, but it does take place in the future when humans have achieved space travel and are fighting an intergalactic war.
We're not the only ones out there, and instead of living peacefully, the universe is basically a free-for-all between the different species trying to establish new colonies on habitable planets.
But for recruits, they don't want young soldiers who may be physically fit, but lack the experience the Colonial Defense Force (CDF) desires. As a result, you can't join the army until you are 75. Reportedly they can make your body younger and battle-ready, though no one on Earth knows how.
This was a fun and exciting book and I do plan on reading the rest of the series.