Groot is going through some growth spurts this year between May's Guardians of the Galaxy Vol. 2 (and that post-credits scene) and the Avengers: Infinity War trailer that has been shown at D23 Expo and Comic-Con International: San Diego (and leaked online).
Groot to Baby Groot; Baby Groot to... Teen Groot? Kid Groot? Emo Groot? Hipster Groot?
Gunn does his best to explain it all - and the proper terminology - on Facebook without going into spoiler-territory.
On Groot and His Growth:
Potential SPOILER for those of you who haven't watched Guardians of the Galaxy Vol. 2 all the way to the end, and for those of you who don't want to know anything about the Infinity War trailer that played at San Diego Comic Con and at D23.
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.
Avengers: Infinity War is scheduled for release May 4, 2018.