That explains why I was kind of uncomfy when I watched the Sailor Moon S movie way back when I could access the subtitle version between a teen Luna and an older man like Kakeru (let alone Luna being a cat at the time). I think DIC saw this as well and felt the same way, thus they went for the "Older Luna" route on purpose when they dubbed the series. At least that's why she sounded like that.
There's an interview with Hisashi Kagawa where he seems to say that he didn't know this before working on the S movie, considering how Luna is constantly lecturing and looking after Usagi. Naoko Takeuchi might have intended for them to be the same age, but it never came off like that imo.
I was very nervous working on "Bishoujo Senshi Sailor Moon S The Movie Kaguya Hime no Koibito" as it was my first time as a movie animation director. I found Takeuchi-sensei's idea to humanize Luna to be interesting. I also thought that Takeuchi-sensei's character draft was "very cute." My image of Luna was older than Usagi and the others, and I didn't think of her as feminine. Luna herself feels like a caretaker in the series, and there were scenes where she preached to Usagi-chan.
While they might be supposed to look younger, they are actually very old creatures. Plus they are aliens. I guess they stopped aging once they hit let's say 15 but their brains were developijg and maturing, unlike Chibi Usa that stopped growing while very young. I read somewhere that Helios is supposed to look like a 12 year old. But then all the girls in the manga look way older than they are.
I think I agree with her aging in cat years but not having the same life expectancy of Earth cats because she is an alien one, perhaps?
With her being the same age as Usagi currently, Luna is either in her 90's or was given to Princess Serenity in the past and put in cryo when she was just 2 years old, considering Serenity herself died at 14.
Of course, Luna may be very smart though considering she's an alien cat but I think she had to have many years of experience and exposure to learn all that she knows.
i think is much like the 90s anime case, in the manga they are the same age even you see that in some interaction, but in the 90s you think luna is much older because of her behavior thats what happen to with mamoru, in the 90s you see how mamoru acted in thing he is much more older than usagi