Honestly, my reasoning is mostly along the lines that, excepting Ruby herself, all the first years we've seen in the show have been 17. I'm not really against first year students being 18, but it seems to me that based on what little we've seen of the school system most if not all first years are 17. If you really want to have a character be older, I don't think it's that hard to add a sentence explaining why they got into Beacon late.