I think we've discussed how to be perceived as older pretty well
here.
Make sure the polo fits well and change up your bottoms. Try wearing different pant silhouettes over heels? Any chance you can add sweaters/cardigans/blazers? Also, I think the trick working with this uniform is to keep up with the grooming (make-up & hair) and add accessories - jewelry, scarves, etc., some
flare if you will. Maybe some classic, tasteful pieces, nothing crazy colorful that might pass you off as a teenager...