Most of my favorites have been named, but I didn’t see:
Madeline
The King of Masks (in Chinese)
was Ever After and Not One Less mentioned?
Nausicaa was my immediate thought upon reading the OP. Porco Rosso is also a good fit, and for a more everyday type of bravery, Whisper of the Heart.
Well, loose fit, but: 9, the 2009 animated film. The only female of the group is 7, and she’s by far the most fearless, and the most skilled warrior. She has a wicked bird-skull helmet, too.
From the same director, Nausicaa of the Valley of the Wind. She’s an incredibly strong character without being aggressive. She is capable of violence but she is not a violent person. She is excellent at handling crisis situations. She does whatever is in the best interests of her people. I adore this movie.
I’ll second True Grit (the Coen brothers version; I haven’t seen the other one and don’t plan to, based on what I’ve heard). Given the 18-and-younger deal, consider Crouching Tiger, Hidden Dragon for a very brave and kickass girl.
Bridge to Terabithia
Candleshoe
The Bad News Bears
Paper Moon
Whistle Down the Wind
The Trouble With Angels
Meet Me In St. Louis (the younger sisters are awesome)
The Secret Garden (1949 or 1993 (theatrical as opposed to TV versions))
Matilda
The Borrowers (1997)
The Secret Life of Bees
Peter Pan (2003)
Adventures in Babysitting
My Girl
Beetlejuice
Bend it Like Beckham
Heathers
Hairspray (either version)
Ponette
Forbidden Games
London to Brighton
Most of these are just films with central girl characters; I’m not sure they do anything heroic or brave besides maybe standing up to authority/bullies, show they’re as good as boys, or just get past their own fears. There are plenty more that I can think of, but a lot of them tend to have the same young actresses in them (Hayley Mills, Jodie Foster, Dakota Fanning, Christina Ricci, Wynona Ryder…).
If the OP was taking TV shows, I’d suggest Nadia: The Secret of Blue Water.
The City of Lost Children
Any of the One Piece films
I can’t remember the name of the little girl, but she was pretty involved with the events throughout the movie. I wouldn’t be surprised if she has more screen time than any of the member’s of the Baron’s team.
Sarah Salt, played by a young Sarah Polley and she is basically the protagonist of the movie.
I’ve never seen this movie, but Island of the Blue Dolphins is one of my favorite young adult books, and I just thought I’d check to see if there was ever a movie, and there was.
Dot and the Kangaroo, perhaps? I don’t remember much of it aside from that super-creepy bunyip sequence, but I think it fits the bill.
What about Nancy the protagonist of the original A Nightmare On Elm Street? She certainly isn’t a coward, and is the only one brave enough to take on Kruger.
Ditto for Wes Craven’s New Nightmare where the actress is playing both herself and Nancy(yea it is a fourth wall/reality warping kinda movie) she has to save herself and her son.
Anything by Joss Whedon.
StG
Which reminds me that the movie Buffy the Vampire Slayer might fit the criteria. Theoretically 18 or so, kick-ass, and only very mildly sexualized.
Whedon’s Buffy TV series has all kinds of brave and/or butt-kicking under-18 females. They’re thinner on the ground in Angel, I think, and Firefly/Serenity definitely have strong and non-stereotyped females, but the only girl is a combo of butt-kicker and (not-her-fault) crazy, so I’m not sure I’d describe her as ‘brave’.
It blew my mind that Nausicaa didn’t show up until page 2. Like Sir Not Appearing In This Film, that was my immediate thought upon opening this thread.
Since Hanna is cited in the OP, can I suggest Léon/The Professional?
Is Noa Izumi from Patlabor (the first movie is fantastic!) too old to qualify?
Matilda. Danny Devito’s film adaptation based on the book by Roald Dahl.