I also have no knowledge on the real meaning. But based on my observation, it is always to refer either to real old woman, married woman or older woman regardless of there's family relationship or not. Just like some younger girls call older guys as ahjushi. Ahjumma and ahjusshi are also a common used to call the sellers in the marketplace.

On "doesn't it get lead to actresses getting discriminated against in the acting industry" - many actors /actressess who are 40s above normally will only keep playing the role of father /mother and maybe just supporting roles, but that's not all. Some more established actors can still headlining drama/movies in that age. I don't know about discrimination in the acting industry, but the fear from being labelled as an ahjumma and getting discriminated because of it, is probably why some actressess are quite obsess to maintain their youthful look.

The word "ahjumma" in Korean literally means "aunt" - it is in fact an older archaic term that has lost it's literal meaning and is used to convey a wide range of different things. What people object to is the connotation of the term depending on the social context. Bit of an oversimplification, but consider the term "ma'am" (or "sir") - and the reactions that can arise from being called such (particularly if the person in question is not eager to relinquish her/his identity as a "young" person). Here endeth the lesson- from someone who is now definitely an "ajusshi" (similarly, literally "uncle")