In The Wolverine, Mariko jokes that she was going to marry Harada, but couldn't because they weren't fifteen. By the time they're adults, however, Mariko has to enter Arranged Marriage with Noburo Mori.
Sky High (2005): Layla and Will are neighbors and because of that and other things Warren believes this of them. It becomes a fact at the end of the film.
The Spider-Man Trilogy: Peter Parker and Mary Jane Watson knew each other since they were kids, and they start going out when they are young adults, which is helped along when MJ learns that Peter is Spider-Man in the second film.
The Curious Case of Benjamin Button is a curious case indeed. Benjamin Button was born as an old man and so his "childhood" with Daisy Fuller looked like a girl hanging out with her grandfather. Nevertheless, they're a couple when they're the same age.
Forrest Gump: Forrest and Jenny walked to school together every morning. It's used as a time skip it's such a long duration. It's a bumby ride but they are ultimately Happily Married. Then, she dies.
Melody(1971) took this to extremes with the ten-year-old lovebirds deciding to skip the part about growing up first and get married at once. They pull this off, albeit unofficially, and it turns into an elopement when the adults raid their wedding party.
Star Wars: Anakin Skywalker marries Padmé Amidala, whom he met as a child.
Abby and Jay fall into this near the end of 16 Wishes.
In Some Kind of Wonderful Keith and Watts have known each other since at least the third grade. Initially Keith doesn't notice her—due not only to knowing her for a very long time, but also because she's this close to qualifying for Wholesome Crossdresser—but following a whole lot of Character Development for everybody and a Practice Kiss, he ends up choosing her. Curiously enough, the popular girl he was pining after isn't made out to be a bad person, just one who's better off on her own.
When Mary of It's a Wonderful Life was a child she said that she was going to marry George. He didn't fulfill his dreams but she did before too long.
13 Going on 30: Jenna and Matt were best friends in their childhood as seen in the beginning of the film. Their friendship broke off when Jenna blamed Matt for pulling a practical joke on her during a game of 17 Minutes in Heaven. Years later, they reconcile and develop romantic feelings for each other. They got married by the film's end.
Thor: The Dark World: It was evident in Thor that the titular character and Sif, along with the Warriors Three, are very close friends, merely hinting that Sif might have romantic interest in Thor. The second movie then heavily implies Sif will turn out to be an Unlucky Childhood Friend. She attempts to reach out to Thor while he is still brooding about not being able to see Jane, but he just thanks her for her friendship and skill in battle and walks away. His father Odin tries to set him up with her later in the movie, but he replies she is just a friend. Throughout the rest of the movie, he repeatedly ignores her in Jane's favor, and during the few times they do interact, it appears to be purely platonic on his side.
Jaimie Alexander: She (Sif) does sacrifice quite a bit for Thor, because she is so very much in love with him, so you do get to see that... She cares about him. They grew up together, you know?
In French movie Love Me If You Dare, the lead couple grew up as kids together, sleeping together in the same bed well into their teenage years and they continue acting out their Belligerent Sexual Tension into adulthood.
In musical comedy, The Fighting Temptations, the lead couple — Darrin & Lilly (portrayed by Cuba Gooding, Jr. and Beyoncé Knowles as adults) briefly spent their childhood together before Darrin moved away with his mother. Darrin would often ask Lilly to be his girlfriend but she'd decline as she had her heart set on singer Michael Jackson. Ironically, they reunited in their adult years but Lilly had no romantic interest in Darrin whatsoever until late in the film. He proposes to her in one of the last scenes of the movie, and it is later revealed that they had a baby of their own.
Neverwas includes a burgeoning relationship between Zach and Maggie, who had apparently romped around the neighborhood in grand adventures as children, although Zach had largely forgotten her until they met as adults.
Jonas and Fiona are this in The Giver due to the latter having been Promoted to Love Interest from her role in the book — there's even a montage shown in pictures of how close they'd been since children before they start making out.
Wesley and Bianca in The Duff. They were next door neighbours and grew up together, until Wes became a Dumb Jock which annoyed the hell out of Bianca. After Bianca got transformed from a shy DUFF to a bright beautiful girl, with Wes help nonetheless, the two end up together.
William Wallace and Murron in Braveheart meet as children and become lovers later in life.
Robert Ford and Cynthy Waters are this in I Shot Jesse James. Robert claims he's known Cynthy "ever since I could crawl", and it's his love for her that largely leads to the film's events.
Chiron from Moonlight had an Only Friend in Kevin. As they grow older, their friendship turns romantic.
In Once Upon a Time in America, Noodles and Deborah are childhood friends who flirt. They don't see each other for the rest of their childhood/adolescence after Noodles gets sent to prison, but still have a romantic connection when they see each other again as adults.