In “Guardians of the Galaxy Vol. 2,” we get to see Baby Groot, who resulted from the little bit of the original Groot that Rocket planted at the end of the first film. This allowed us to look at the life cycle of one of his species more thoroughly, as we traced the character’s growth through “Infinity War,” “Endgame,” “Thor: Love and Thunder,” and “Guardians of the Galaxy Vol. 3.” We left off in a pretty interesting place with Groot most recently in the credits scene attached to that film.
But throughout the character’s evolution on screen, the aging process has never been precisely explained. The answer, it seems, lies with James Gunn himself, who actually offered a pretty good explanation in 2017 around the time that “Vol. 2” was in theaters. Taking to his personal Facebook page, the filmmaker laid out how Groot’s aging process actually works within the MCU.
“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.”
So there we have it! “Infinity War” Groot could easily be the equivalent of say, a 12-year-old, while the 2014 Groot could have been more akin to a 19-year-old, even if the human math doesn’t exactly line up.