My story is set in ancient Sumer, and I have a teenage (side) character who has a typical temperament meant to evoke his age. Had the setting been more modern, I would have had him use slurred together words like "Wassup?" and a light dose of slang. However, both of those may sound too modern for the age, and pull the reader straight out of the 2250 BC setting. I have gotten this feedback from a friend of mine before.
What can I do to alleviate this issue, and are there alternative tactics? I would also be interested in other ways to make teenagers sound teenage, that are independent of time setting.