I’ll take your word that your comments stem from wanting a more “comic book accurate” Supergirl. The character was created 60 years ago for the purpose of showing Superman as a father figure, things have thankfully changed a lot since then. While Kara is depicted as chronologically younger than her cousin certainly not all versions have been particularly young and defiantly not innocent. The current comic book version even had a conversation with her cousin where she pointed out she’s actually older than him.
Some of your comments especially without context can be mistaken as sexist, using different words would help. Add some context if you like picture and be specific just like you would if you were giving someone a complement in real life. As an example you could say that you like how Melissa looks without makeup because you like the no makeup look on a woman. When a man says they like it when a woman looks younger that brings certain stereotypes to mind. The cliché of a man leaving his wife for someone who looks like younger version of her is around for a reason. Innocent is not a term most woman want applied to them. Some adjectives of innocent are naïve which no adult wants to be described as and pure which has sexist connotations when applied to a woman.
Helen Slater was 19/20 when she filmed the movie, so she was pretty much the age of the version of the character she was playing, and it was her first role. I’ve never seen Grease the movie, so I can’t comment on that. I do know it was made 40 years ago and the Supergirl movie was 35 years ago and expatiations that a woman should look and act a certain way have changed since then. I fully realize that makeup and lighting can change the age an actor looks and then there’s the fact that some people simply look younger or older than their chronological age. The current trend is to have actors play characters not too far from their actual age and to have better roles for actors of all ages especially women.
The version of Kara on this show is a grown woman who while she's navigating a complicated life is smart and capable. She’s optimistic and hopeful but she’s not innocent. For me there’s no reason to make Melissa, who yes does look younger than her age, look younger for the role. If the rumors about the movie are true that Kara will be a younger (17-19) version of the character. That may be more like the version of the character you prefer as it would be more like Helen Slater movie version. If that’s the case I believe casting an actress in her late 20s is questionable but dependent on the individual.
The context partly follows on from previous posts that I or other posters have been discussing previously, going back to Melissa with her bangs looking cute, or people saying how sweet she looks.
I find it rather disturbing that you gloss over that, and instead your thoughts automatically go to the worst possible interpretations of the words "young" and "innocent". You see the word "young" and by default you suddenly think of people trading in their wives or girlfriends for younger models, or you see the word "innocent" and immediately you think of naive. That would never have even crossed my mind.
Young can also mean youthful and fresh-faced looking, which is often what grown women do like to be thought of rather than having someone say to them "you look your age or much older". And innocent doesn't have to just mean naive. It can also mean earnest, wholesome, good, pure or even guilt-free. Or within the context of when applied specifically to a woman, it can mean girl-next-doorish or approachable. It doesn't have to just mean naive or gullible or even potentially clueless.
Before the Wonder Woman movie came out in theatres, people were debating whether Gal Gadot was suitable or not. One of her qualities which I praised and still do is the fact that she has an innocent look. I make no apologies for that. Now does that mean that I'm praising that because I think she looks naive or stupid? No. It's that she has those qualities I've just mentioned above, and that she also doesn't have a resting ***** face, or even a scowling ***** face when showing her determination and toughness.
Now maybe your own personal experience of the words "young" or "innocent" have been negative. However, that doesn't mean that those are the default interpretations of those words or that they automatically have to equate to negative stereotypes. Otherwise we could apply that to many other words and we'd have to start second guessing other people's interpretation of them all the time.
It's like if someone hears a saxophone and immediately equates that with sleazy porn music rather than something far more positive like jazz, which some people do. You have to wonder then what that actually says about the person making that association and if the reason they automatically make that association is because they've been overexposed to that. For anyone else who has listened to jazz, a negative association like porn music wouldn't even come to mind when they hear a saxophone.