Young Centurion?

[Marcus Julius]

... Besides at that age you are more reckless (heroic) and cruel (very handy in those days). The ss used very young boys too for that reason

I have to both agree, as well as disagree with that. The 'juegend' began training at a young age because they are so much more impressionable. It is easier to brainwash them into fanaticism when you start earlier.
Now for the agreeing part - it may have been beneficial to Rome to have such young minds in the post of centurion at a young age for similar reasons. They could be tested and taught by higher officers, and conditioned to be loyal to the command - as opposed to someone who rises from the legionary ranks who would have his loyalties to the troops with whom he had once served.