What if the part after "depuis" requires l'imparfait or passé composé?

Bonjour Brittany, It seems to me that in your first phrase «Je joue du piano depuis j'avais six ans» --> J'ai joué au piano depuis j'avais six ans -- would be more correct by using the passé composé along with l'imparfait; however, since playing the piano is a habit or something learned that is still ongoing, I would rephrase it to be Je jouais du piano depuis j'ai eu six ans due to the fact that playing the piano is a habit, learned skill that is continuing. One caveat here, like a former French teacher told the class, the French language is not simply English translated into French. And your second phrase, I believe is the most grammatically correct. Bonne chance,