Baby Groot is easily the most adorable (or the most detested, based on who you ask) character in Guardians of the Galaxy Vol. 2.
[Warning: The following contains spoilers for Guardians of the Galaxy Vol. 2 and the Avengers: Infinity War trailer.]
If you stuck around for the multiple post-credits scenes in Guardians of the Galaxy Vol. 2 and managed to get a glimpse of the Avengers: Infinity War trailer, you would have noticed an adorably bored, video game-obsessed teenage Groot. In Guardians of the Galaxy Vol. 2, he’s being yelled at by Star-Lord. In Avengers: Infinity War trailer, he’s playing a handheld, ‘80s-inspired video game while the rest of the Guardians look upon a comatose Thor.
Not much time has passed between Guardians of the Galaxy Vol. 2 and Infinity War, which both take place canonically a few years apart. That’s a pretty big growth spurt for a tiny tree, leaving many people wondering just how fast Groot grows. It became a big enough question that director James Gunn decided to go into great detail on Facebook explaining just how fast Groot ages.
In terms of how fast Groot grows overall, I can tell you this: Baby Groot appears two months after the Infant Groot we see at the end of Vol. 1. The post-credits scene with Adolescent Groot and Peter Quill is YEARS after the ending of Vol. 2. I won't say exactly how many years at this time, but be clear that it took him a fair amount of time just to go from Baby Groot to Adolescent Groot. (As a side note, I'll point out that Peter Quill can obviously understand what "I am Groot" means in the post-credits scene, so somehow he's learned to understand him in the same way Rocket has).
Like dogs, who grow at a different rate than humans, and spend a different percentage of their lives in adolescence than humans do, Groot and Humans do not grow at some exact exchange rate. So, although two months seems to be about two years (as Baby Groot seems to behave roughly like a two year old), it is not fair to assume that every month is a year to Groot. He has an alien biology and ages in a completely different way than a human being. Different aspects of his growth may take longer or shorter than a human's.
Based on Gunn’s description, it seems that the version of Groot in the Guardians of the Galaxy Vol. 2 post-credits scene was a pre-teen version. In Avengers: Infinity War he’s officially a teenager. This new information lends itself to another important question: Will we see the return of adult Groot in Guardians of the Galaxy Vol. 3?
If Groot is aging that quickly — and seemingly getting older with each new Avengers and Guardians installment — it seems pretty likely that we’ll see him back sooner than we might have imagined.
Teen Groot will join the rest of his Guardians teammates in their biggest battle yet in Avengers: Infinity War when it’s released on May 4, 2018.