Board Thread:Off Topic/@comment-30582209-20170214224204/@comment-26397825-20170222031259

Because most people who write fanfiction are young teens themselves and want their characters to interact with RWBY themselves, so they put them at around the same age to give them a 'believable' reason to do so.

And because some people are afraid of dipping their toes into uncharted territory, which is what writing a story about a graduated Huntsman in the bigger world would be, especially since we know practically nothing about the world of Remnant. Anything they write can be instantly disproven with a new episode, and then they'd have to either soldier on with their AU, retcon things to make it fit, or just give up entirely.