I think you are on the right track.
It has occured to me through an analogy to my native language (which is not a romance language, but often works far better than English for understanding Italian) that ancora means still in positive semtences, but is better translated as yet in negative sentences and probably questions.
Based on what yuioyuio says above this sentence should be 'not yet' or 'not even'. I agree with you that 'still not' is not an English way to say this. I am still not sure whether this really is the intended meaning. I would love an Italian speaker to confirm that the meaning is that she is younger than 30.
I originally answered this with "she is not still 30 years old" - my meaning being she is older than that now. Since reading the comments I've realised I've read it wrong. It means she in younger than 30, and my "still" is in the wrong place. The problem is, my answer was accepted. Had I not checked the comments I would have gone on believing this could mean something it cannot, right? In no situation could "lei non ha ancora trent'anni" mean "she is no longer 30 years old", sì?