I don't think expecting a 15yo to do one task is unreasonable but I will offer a but...
Growing up, neither my sister or I had any jobs to do. Mum and dad did everything or the woman who looked after us did everything.
Later on, when I was 13, my mum got sick and took time off work. Sister and I were both expected to do jobs - both of us resisted (I'm not proud of this at all)
I don't think teens doing jobs is unreasonable at all, but it depends what happened in the years before they turned 15, how easily they'll take to doing those jobs.