In comic #1418, Hannelore mentions finding a box of pocky under Marigold's bed from ten years ago. So unless she actually made a conscious effort to keep the pocky (which wouldn't really fit in with her character unless she had some kind of hoarding mental disorder), she would've had to live in that apartment for at least ten years. Now given that the youngest age she could've moved out when she was a kid was 16, that makes her at least 26. However, this is assuming that she actually did that, also that she never moved. Assuming she waited until she was 18 like most people, she'd be at least 28. Even so, that's assuming she either never went to college or lived in one apartment during and beyond college. If you don't make that assumption, she'd have to be in her thirties, possibly older. Yet, of course, she looks like she couldn't be older than maybe, maybe 25, especially with the acne.