The drama is a midnight drama so probably starts at around 24.
There's a huge difference between actresses and idol actresses. An actress, let's say Miyazaki Aoi is no way near the same as Mano Erina. Mano is an idol doing photobooks, radio, acting, variety (albeit not much), singing, concerts, etc etc. While Miyazaki Aoi is an actresses and simply only does that. She appears in magazines and has radio shows (I think) but that'd be it.
Because of this, people seldom think much of idol actresses as it's very, very common they get put in dramas/movies/plays for other reasons than their acting abilities. Now, it turns out that Mano has actually gotten pretty good over the years so she has very high potential to get to become a real actress imo. But some people get stuck with their idol image forever.