James Gunn has explained why the everyone’s favorite Groot was an adolescent in the upcoming Avengers: Infinity Wars. He said:
“People are asking me a lot about how fast Groot grows, because in his appearance in the Infinity War trailer he was an ADOLESCENT (not a “teen” as has been reported in many places). This is exactly the same state of growth you see in the post credits scene of Vol. 2: Adolescent Groot.
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.
And, other than that, I’m going to leave you in suspense – frankly, for quite a while, as it will take years to show you how Groot goes from infancy to adulthood.”
Source: James Gunn Facebook