One thing that is easy to forget about medieval settings is that people generally grow up faster. Whereas someone in today's society can be considered "not adult" at 18-20, it wasn't unusual in the middle ages to have people go from being considered "children" to being considered "adult" at the point of biological adulthood. That point is the point of sexual maturity, so someone could easily be considered an adult around the ages 12-14.
Children, especially in non-noble families, were expected to work from when they first could do it. (around ages 5-8, according to one source we checked). As such, the concept "teenage" is not really something that exists in a medieval setting.

If you're in doubt: we have no problems whatsoever with you playing whatever age you think makes sense, so long as you can play it plausibly.