That's because due to some new virtual tech that Tony is demonstrating, fans are able to see a Tony much younger than his present day self. That was achieved thanks to the team at Lola VFX.
In an interview with THR, Lola VFX Visual Effects Supervisor Trent Claus explained some of the base work that went into making him appear so young. It all still starts with a performance from Downey. "Instead of completely replacing the actor with a digital double, this method allowed us to retain the actor's performance and nuances. Then we began to adjust the on-set footage of Tony Stark through digital compositing."
Claus continued: "Every feature of the face and body needed to be addressed in some fashion. One thing that happens to all of us is that the skin of the face gradually lowers in certain areas, and needs to be 'lifted' back to where it was at the age in question. But other changes are incredibly subtle, such as increase in the way light reflects off the sheen of the skin, a reduction in the appearance of tiny blood vessels under the surface of some parts of the face, or more blood flow in the cheeks giving them that familiar youthful 'glow."
Making it even more challenging though is the fact that Downey Jr. is so recognizable. "Additionally, when working with the appearance of a well-known actor such as Robert Downey Jr., there is the added pressure of living up to the youthful appearance that audiences remember," he says. "In this case, we analyzed footage of Mr. Downey at the approximate age that we wanted to target, which was around the time of the film Less Than Zero [when Downey was in his early 20s]."
The tech is certainly impressive, and overall the team did a fantastic job at making young Tony look lifelike and not like a clear CGI stand-in like so many others end up being.