To be fair, Grease is a parody, and that was part of what they were parodying at the time. So they went a little extra with it too.

It's a parody of the super common "beach fling" style of movies from the 60's. They even reference some common actors and actresses from those movies.

It's why each scene seems so poorly tied together, and the characters aren't very consistent.

Sometime the reverse is true.

Keira Knightley had just turned 18 when she filmed Love Actually (2003). She played a newlywed woman in her mid to late twenties. Her famous "floppy hat" was used because she had a giant teenager zit on her forehead.

Sometimes the casting is dead on.

Mariel Hemingway was 16 when she played the 17 year old love interest to Woody Allen's 44 year old man in Manhattan.

I think, in movies, a big driver in casting is trying to guide the audience. Had they cast actual teenagers in Grease, the audience would have been scandalized by the subject matter rather than focus on the comedy and music. That's why they cast a teen to play a teen in Manhattan. The entire thing is meant to feel off and uncomfortable, which would not have been apparent had they cast a woman in her mid twenties. They cast a teen Keira Knightley because they needed the audience to instantly understand why the character had a crush on her. Whether we like to admit it or not, our monkey brains register teen women simultaneously as angelic/pure and "peak breeding material" (yes, it's yuck when put that way).

Movie magic is built on multiple layers of subconscious manipulation.

Just like my boy Jotaro Kujo

So is the dude next him. Neither are passible.

That's what you get for smoking as a teenager.

Vsauce - Did People Used To Look Older? https://www.youtube.com/watch?v=vjqt8T3tJIE

Michael Tucci, born in '46. He was 32 here.

He looks like Rodney Dangerfield.

Don't smoke, kids, or you'll look like this 😂

I was able to grow a full beard in 10th grade so this isn’t that unrealistic.

JAV: first time?.jpg