There are some things you can do to "dress older". My sister dressed in very "serious" clothes in her twenties and people often didn't fully realize how young she was. But I still have people think I am about 5 years younger than I am. Given that I get viewed as younger than I really am even on the internet, I think it has a behavioral/attitudinal component and isn't just about looks. I have read of examples in various books of taking a cute, petite woman and dressing her much more seriously in order to overcome the cuteness factor and get her taken seriously.
On the other hand, my 22 year old son gets misread as being a lot younger than he is and he doesn't hesitate to take advantage of such assumptions. Just tossing that out there as food for thought: Maybe it isn't only a problem. Maybe there is also an opportunity in there.