Aliens vs. Predator: Requiem: Jesse and Ricky talk about how they hung out together once and did stuff like sneak into the school pool "before you had a boyfriend." In the present, Ricky has a crush on Jesse which he clearly hasn't voiced before. Jesse easily notices this and proposes a Relationship Upgrade after she dumps her Jerkass boyfriend.
Stevie and her ex-fiancé from Backstreet Dreams were childhood friends before they got engaged. They still split up because he didn't understand why she was willing to work for little or no pay.
William Wallace and Murron in Braveheart meet as children and become lovers later in life.
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.
The Dark Knight: Rachel breaks her promise to wait for Bruce in Batman Begins and ends up with Harvey "The Two-Face" Dent instead.
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.
Erin's Guide to Kissing Girls: Erin and Liz first met in kindergarten, and were best friends ever since. After Erin gets rejected by Sydni Hunter, Liz stands up for her, and they share a dance in the school hallway, signifying that they have become an Official Couple.
In Esteros, Matias and Jeromino used to be childhood friends who also experimented sexually with each other. When they meet again many years later as adults, they're forced to confront that their feelings for each other never truly went away.
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.
Flipped: Juli meets Bryce since he moves in across the street from her and she instantly falls for him. But, Bryce is utterly annoyed by her stalking habit that he does everything to keep Juli at bay. They almost end up as Unlucky Childhood Friend until Bryce decides to change his ways toward Juli.
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.
Get Married If You Can: Dani has been in love with Gustavo since she was seven years old. He initially is infatuated with her sister Ana Paola but later realizes it’s always been Dani.
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.
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.
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.
In Jumanji, Alan Parish and Sarah Whittle were friends as children before the titular game sucked Alan in for twenty-six years, after which they were reunited and grew close over the course of the film before the Reset Button gave them back their childhood. In the epilogue, their new future has them married and expecting a child.
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.
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.
Chiron from Moonlight had an Only Friend in Kevin. As they grow older, their friendship turns romantic.
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.
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.
Past Lives: Nora and Hae Sung were friends in childhood who reconnect in adulthood. The possibility of their love is discussed by Arthur in the trailer:
Arthur: What a good story this is. Childhood sweethearts who reconnect 20 years later and realize they were meant for each other.
The first scene in Pirates of the Caribbean: The Curse of the Black Pearl is Will Turner and Elizabeth Swann as children meeting, when Will is saved from a shipwreck and taken in by the crew of the ship Elizabeth's in. Skip forward to the main story and Elizabeth considers them close enough for First-Name Basis. Will, for his part, has this Always Save the Girl compulsion that Jack never lets him live down. They marry in the third film.
In The Silence of Adultery, Rachel and Paul have been friends since kindergarten and fell in love once their hormones kicked in. That didn't stop Paul from becoming Married to the Job and the love from going out of their marriage.
Abby and Jay fall into this near the end of 16 Wishes.
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.
Jamal Malik and Latika in Slumdog Millionaire met each other as young children and developed a deep; deep bond—to the point that once separated from each other, Jamal later found and rescued Latika when they were teenagers and then reunited with her once again as adults where he's fighting even harder than ever to free and rescue her.
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.
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.
Star Wars: Anakin Skywalker marries Padmé Amidala, whom he met as a child.
Superman: The Movie: A deleted scene reveals that Lois Lane was the admiring young girl on the train watching when young Clark Kent was running alongside it.
H.W. Plainview and Mary Sunday end up together as adults in There Will Be Blood after having become good friends when they were both young children early in the film.
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?
Raven for Xavier in X-Men: First Class. Xavier does a lot of flirting with other women, and Raven is clearly jealous. It's hard to tell whether she actually had strong romantic feelings for him, or if she just wants to establish that she's worthy of romantic interest, but either way he says that he can't see her as anything but a sister and someone to protect.
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.
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.