Childhood Friends

Childhood friends is a trope concerning two (or more) characters that have known each other closely since a young age. Their relationship may or may not become romantic in later life.
Romantic works often follow how childhood friends get together, making the transition from Friends to lovers, and explore the challenges that come with changing a long-held relationship. The situation may be that the friends didn't know they were dating. The friends may have been hiding romantic feelings and pining, or come to the realization that their feelings are more than platonic.
==Canon examples==
*Fullmetal Alchemist : Alphonse Elric, Edward Elric, and Winry Rockbell are childhood friends (Ed/Winry is a canon ship)
* Marvel: Steve Rogers and Bucky Barnes (Stucky)
*Free!: Haruka Nanase, Makoto Tachibana, Rin Matsuoka (MakoHaru, RinHaru, MakoRin)
*Haikyuu:
**Oikawa Tooru and Iwaizumi Hajime (Iwaoi)
** Tsukishima Kei and Yamaguchi Tadashi (Tsukkiyama)
**Kuroo Tetsuro and Kenma Kozume (Kuroken)
**Sawamura Daichi, Sugawara Koushi, and Azumane Asahi (Daisuga is popular)
* Boku no Hero Academia: Izuku Midoriya and Katsuki Bakugou are childhood friends-turned-rivals (Katsuki Bakugou/Midoriya Izuku)
* Assassin's Creed III: Kanen'tó:kon and Ratonhnhaké:ton are childhood friends.
* The Fire Emblem series has many pairs, roughly half of them romantic in nature. Most notably:
** Marth/Caeda in the Akaneia series
** Alm/Celica in Gaiden and its remake Echoes
** Azelle and Lex as a platonic example, either one with Tailtiu as romantic in the first generation of the Jugdral games. The second generation has Leif/Nanna and Seliph, Lana, Larcei, Scathach, Lester, and Diarmud.
** Roy/Lilina and Wil/Rebecca as romantic examples in the Elibe games. Hector and Eliwood are a platonic version.
** Ephraim, Eirika, and Lyon are a tragic example in The Sacred Stones. Colm/Neimi is a romantic example.
** The Tellius games have Ike/Soren, Mist/Boyd, and Geoffrey/Elincia as romantic examples. Mist/Rolf, Aran/Laura, and Lucia/Elincia are platonic, though shipped heavily in the fandom.
** All of the child characters in Awakening can make opposite-sex pairings with each other and are all good friends due to having grown up together. From the first generation, Lissa/Maribelle, Chrom/Maribelle, and Chrom/Sully.
** Princesses Elise and Sakura in Fates are childhood friends with their retainers and can marry the male ones. The Avatar is this with the maids Flora and Felicia and the butler Jakob.
** Ingrid with either Sylvain, Felix, or Prince Dimitri; Yuri/Bernadetta, and Jeritza/Constance in Three Houses. Linhardt/Caspar isn't explicitly romantic, but Linhardt is canonically bisexual and has a paired ending with Caspar.
* In Kim Possible, the titular character and Ron Stoppable are childhood friends who become lovers.
* Kevin Arnold, Winnie Cooper, and Paul Pfeiffer in The Wonder Years are a trio of childhood friends.
* Cory Matthews, Shawn Hunter, and Topanga Lawrence in Boy Meets World.
* Miaka Yuuki and Yui Hongo in Fushigi Yuugi, the main driving point of the series' plot has them becoming enemies.
* Ash Ketchum and Serena in the Pokémon XY anime were retconned to be this. Ash and Gary are childhood friends turned rivals turned friends again. Dawn has a childhood friend named Kenny in the Diamond and Pearl series.
* Kamui Shirou and the siblings Kotori and Fuuma Monou in X/1999
* Tomoyo Daidouji and Sakura Kinomoto in Cardcaptor Sakura
* Pathologic: Apple Basket Gang in Pathologic 2
* Phantom of the Opera: Raoul de Chagny and Christine Daaé are childhood friends who end up a couple
==Fanwork examples==
*Would you, Won't you by Mad_Merry. Desmond Miles/Delsin Rowe childhood friends AU
==Additional resources==
*"Childhood Friends" tag on AO3
*"Alternate Universe - Childhood Friends" tag on AO3