In a science-fiction setting, with warfare loosely inspired by the second world war, there exists Foo. Foo are a species that are very similar to humans, but they biologically stop aging between their late 20s and early 30s (but continue to have lifespans similar to humans, as described in this question).
Due to cultural norms and Foo's species loyalty, retirement is a very uncommon phenomenon. Further Foo's mental biology has natural means of repairing itself over time to a mentally conceptually ideal state, allowing the brain to recover from some mental disorders including posttraumatic stress disorder.
These traits, collectively, allow a Foo to be a career soldier, serving from 18 until death as an active combatant.*
*As a minor note, for Americans serving in the armed forces, military death only increases likelihood by 2.5x as compared to a normal citizen (Preston and Buzzel, 2007), so combatant turnover isn't as steep as one might expect.
Given the realization that the military expects to get multiple decades of service out of a given individual after training, how would this affect training on terms of length and specialization? How would the training differ from our military training of humans, here on Earth?