I hadn’t thought of it that way before, but that’s a good explanation.
From WIKI:
Qualifications for becoming a Centurion
A man in Roman army who wanted to become a Centurion had to meet many qualifications. First, the man had to have several letters of recommendation from important people. Therefore, it would help if one trying for the position was befriended to several important senators, or even the consuls/emperor themselves/himself (depending on the time period).
[edit] Age Requirements
A man who desired to be a Centurion would have had to have been at least thirty years of age. Therefore, if he had entered the army at age 16, he would have spent almost half his life in the military, giving him the experience he needed so that he would know how to sufficiently command his cohort.
[edit] Social Status
As stated above, the aspiring centurion needed good connections to be recommended for the post. These connections might have been easier to achieve if the centurion was of a higher class.
[edit] Education
One of the tasks of the centurion was to relate his superior officers’ written commands to the men in his cohort. Therefore, he had to be able to read. In order to do this, he needed at least enough education to be literate. Also, the more educated one was, the better chance he had of becoming centurion.