Ive owned and read and re-read the 2nd edition of WFRP for a few years now but unfortunatly not had a serious game due to lack of players. Seeing as ive been aware of the game and understood it since i was 14 i can give a younger opinion to the question. I have a fairly detailed knowledge of the Old World as ive been playing Warhammer since i was 7 and i know other people around my age group who share my understanding.
That said the vast majority of mid teens would not have the patience or imagination to understand or appreciate the more complex and detailed sections of WFRP. I think it is not the age so much as the willingness and maturity of the person in question. I know adults who are less mature than me but then i know teenagers who are more so. I believe that under the age of around 12 or 13 you would be hard pressed to find anyone who would easily understand the Warhammer concept in the detail that is required for a mature game of WFRP, though i am only going on speculation.
