The talented digital effects team behind Marvel's Captain America: Civil War had a lot of reference material to draw from when attempting to create a young Tony Stark.
Actor Robert Downey Jr's 80's film catalog consisted of movies like Less Than Zero, Johnny Be Good, Back To School, and Weird Science, and those films gave the effects gurus a great foundation with which to start from.
In an interview with The Wrap, Visual Effects Supervisor Dan DeLeeuw spoke about which film they took the most inspiration from, saying: “We went a little closer to [his look in] ‘Less Than Zero. No one wanted the spikey hair from ‘Weird Science’ like I did!”
Thankfully he was voted down on the hair thing, but at that point, the work was just beginning. The scene required Downey Jr. to act as both his older self and younger self, as towards the end both are in the same frame. A double would act as his younger self first, with Downey Jr. wearing motion capture gear as his old self. After that was done, he then shaved his facial hair, put on the gear again, and acted as his younger self while a double acted out the older Tony part.
It was complicated, but ultimately it all worked out. DeLeeuw said: “The great thing is [Downey’s] been in the business since the age he needed to be in the scene." The team then had to take several passes at de-aging Downey Jr. until they were satisfied. "Each pass is sculpting in a way. Pulling off material, pulling off age … When it feels right, you go back and clean it up.”