WandaVision director Matt Shakman revealed that he deliberately chose to keep Billy and Tommy Maximoff as children for both story reasons and out of respect for their actors.
In an interview with Entertainment Tonight, Shakman was asked whether there was ever any discussion about aging the Maximoff twins into teenagers for the show's final episode, as fan theories had initially speculated. "No, we wanted to keep them at that age," he responded, explaining that "Wanda has obviously missed out on the first 10 years of their life [which] went by really fast, so we didn't want to deprive her of even more time with them. And once we got to know them, we had so little screen time with 10-year-old Billy and Tommy that we wanted to take advantage of what we did have to get to know them really well as people. There's so much innocence there. To have them age up one more time, I think, would have been unfair to those actors and those characters."
Despite being birthed through Wanda Maximoff's powers, WandaVision's fifth episode, "On a Very Special Episode..." revealed that the twins' powers existed outside of her control, including the ability to rapidly age on cue. After she and Vision failed to make them stop crying as babies, the couple's arguments over their situation resulted in Billy and Tommy growing into five-year-olds within a matter of moments. Later, after trying to bargain with their parents about keeping a stray puppy, Wanda claimed that neither child was ready for pet responsibilities until they became ten-year-olds, only for them to simultaneously age up in response.
Theories about Billy and Tommy being recast with older actors in WandaVision stemmed from how the twins' comic counterparts were eventually reincarnated into the bodies of teenagers Billy Kaplan and Tommy Shepard, sometime after being absorbed as children by Mephisto. They would later become core members of the Young Avengers team as Wiccan and Speed, eventually rediscovering their familial connection to Wanda, Vision and each other. A number of Young Avengers members have since been cast for upcoming Phase 4 Marvel projects, including Hailee Steinfeld as Kate Bishop in the Disney+ Hawkeye series, Xochitl Gomez as America Chavez in Doctor Strange in the Multiverse of Madness and Kathryn Newton taking over as Cassie Lang in Ant-Man and the Wasp: Quantumania.
Written by Jac Schaeffer and directed by Matt Shakman, WandaVision stars Elizabeth Olsen as Wanda Maximoff/Scarlet Witch, Paul Bettany as Vision, Randall Park as Agent Jimmy Woo, Kat Dennings as Darcy Lewis, Teyonah Parris as Monica Rambeau and Kathryn Hahn as Agnes. The series is available to stream on Disney+.
Source: ET