Board Thread:General Discussion/@comment-71.37.202.154-20140725182057/@comment-26397825-20160414091022

I always think of them as about 18 or 19 at the oldest. You can call someone a kid even if you're the same age as them after all; if you feel like you are more world weary or more mature than them, you would refer to them as childish or a kid when talking about them with someone else. And Roman refered to them as the kids, which implies Mercury and Emerald aren't physically much older than RWBY themselves.