Back to the Future establishes that Marty McFly and Jennifer Parker knew each other in fourth grade, fell out of contact for a few years, then fell in Love at First Sight in high school in 1984 and began dating.
Lauren Stone was the girl next ship to Nikolai Dante. When she comes back into his life after he becomes a pirate, Nikolai is definitely receptive to rekindling their romance, but unfortunately his relationship with Jena Makarov at the time is, ahem, complicated. Lauren is one of the few young women in the series who Nikolai doesn'thave sex with.
Bamse has the example of former villain Vargen and his childhood crush Virginia, they meet up years later, but by then she is already married.
Black Panther invokes then averts this trope. Nakia, one of the Dorae Milaje (Beloved Ones, sworn bodyguards that were historically also potential brides for the King of Wakanda, though T'Challa does not practice that aspect of the tradition) believes this is what is going on, having grown up around T'Challa. He is tricked into kissing her by an opponent who makes him believe he is reliving an embrace with a (now) ex-lover, confirming (in Nakia's mind) T'Challa's intent to marry her. Her unrequited romance devolves into Stalker with a Crush territory, to the point that she tries to murder his ex-lover and eventually betrays him entirely, pledging herself to Killmonger and becoming the anti-Wakandan terrorist Malice.
Rayek grows up with Leetah, first starts communicating with her magically before she's even born, shares his life with her, and on the day he asks her to stay with him forever... the hero shows up and claims her for himself. (They eventually settle it with a good old fist fight. 10000 years later.)
Mender spends his very early years around The Chief's Daughter Ember, eventually takes her virginity after they reunite, and is promptly forgotten when he goes off to fight in the war and Ember meets a Proud Warrior Race Guy stranger. Elves have no concept of cheating — all sex is good sex — but Ember treats him horribly.
Cutter and Skywise are just enough years apart for Skywise to hit puberty first and Cutter to be left behind wondering why his friend spends so much time in the bushes with Foxfur. By the time Cutter is 16, he loses both of his parents and Skywise loses Foxfur, and it's implied that they spend some time together as mates after that before Cutter ends up with Leetah. They're still occasionally seen sharing a bed together from that point on, but they're never as close again as they were before Leetah came along.
Halo: Blood Line: It's eventually revealed that two members of SPARTAN-II Black Team were in a relationship when they were teen cadets (having known each other in boot camp since they were six). The side effects of their augmentations implies that they don't feel this way about each other anymore, but some of the tension still remains.
Judge Dredd: The story of America Jara and Bennett Beeny in "America". They both grew up in the same city block, but America always had a rebellious spirit while Beeny always considered himself a coward. They moved apart as America joined the democratic movement to end the Judges' totalitarian rule and ended up destitute, while Beeny tried to play it save and ended up a succesful but ultimately unhappy musician. They find each other years later and have a one-night-stand, but America is soon killed during an arrest. Beeny transfers his brain to America's comatose body so they can still be together.
Alas for her, Candide in Save Havens is destined to be the Unlucky Childhood Friend for Leonardo, for a number of reasons, the biggest one being he's Leonardo da Vinci, and he must go back to the past to fulfill his destiny there, and the history books show he never married. She gets a sweet consolation prize though: it's implied that she's the real Mona Lisa.
Envy and Todd from the Scott Pilgrim series were friends since they were eleven, but ended up parting away and growing up seperately. After meeting up again post-college, they hooked up and started a band. Unfortunatlely, both of them have changed and not for the better, though with Todd, this led to him being unfaithful and cheating on her with the drummer, resulting in this subversion.
Supergirl: In the Silver Age, Dick Malverne grew up at the Midvale Orphanage during the same time that Linda Lee alias Supergirl did, and he had a crush on her. Although the two dated for a while they grew apart after college. They met again as adults, and Dick confessed he always loved her and and knew of her secret identity before dying in Young Love.
Superman: In most of versions of the Superman mythos, Lana Lang is Clark Kent's childhood friend who had a huge crush on him. Usually Clark liked her back, but after leaving Smallville, he falls for Lois Lane, and he and Lana become Just Friends.