Marvel Studios' Avengers: Endgame nearly featured a teenaged Thanos at one point in the film's development.
This revelation comes from Marvel Studios Vice President of Development and Creative Director Ryan Meinerding via the newly released book Art of Avengers: Endgame. The tome contains a slew of fun facts and behind-the-scenes information about the record-breaking film, its predecessor Avengers: Infinity War and several other MCU projects.
Included in the book's "Unused Concepts" is information about a scrapped storyline from Infinity War that showed the Mad Titan at various ages. It began with an infant Thanos, progressing through his teenage years, and Meinerdling shared photos of 13-year-old and 17-year-old Thanos on Instagram.
According to Meinderling, the Thanos backstory was going to highlight the character's struggle from his malformed birth through his adolescence. Despite being a pariah, Thanos would travel to planets in a desperate attempt to keep them from overpopulating the universe. When he failed, it would be time to take matters into his own hands (or gauntlet).
Obviously, the visual element of this backstory did not make it into the final cut of the film. Instead, Thanos' motive was described in several lines to characters like Gamora and the group of heroes who met him on Titan. However, while fans were still able to understand his motives, it seems they were robbed of a key opportunity to see Thanos at his most vulnerable.