Just like all babies, Baby Groot won't stay little forever. Already in the two months between the events of Guardians of the Galaxy and Guardians of the Galaxy Vol. 2, Baby Groot has grown quite a bit, going from a tiny sapling to a fully mobile tree. We don't yet know exactly how old Groot will be in Guardians of the Galaxy Vol. 3, but it's safe to say that he won't be the fan-favorite Baby Groot anymore.
A (spoiler alert) post-credits scene in GotG 2 briefly shows Groot as a teenager, but to determine his actual age, you first have to determine when the next movie will take place. Guardians of the Galaxy Vol. 3 was only recently confirmed by writer-director James Gunn, and has no official release date yet. It's believed that the third Guardians movie will take one of the pre-announced release dates for "Untitled" Marvel movies — all three of which are in the year 2020. Granted, just because three years will likely pass between the release of Guardians of the Galaxy Vol. 2 and Vol. 3 doesn't mean three years will pass for the characters between the films. Films in the Marvel Cinematic Universe usually match up chronologically with the years of release — for example, the first Guardians film took place in 2014, the same year the movie was released. But, the sequel is already changing those rules, which means Vol. 3 could be similarly paced.
The timeline for when Guardians of the Galaxy Vol. 3 will take place has yet to be confirmed, but in his GotG 3 announcement Facebook post, Gunn confirmed that the film would take place after Avengers: Infinity War. Marvel Studios President Kevin Feige, meanwhile, confirmed in an interview with CinemaBlend that Avengers: Infinity War, set for release in 2018, will take place four years after Guardians of the Galaxy Vol. 2.
Guardians of the Galaxy Vol. 2 takes place two months after the original film, and in GotG 2, it appears that he is really more of a Toddler Groot, so somewhere around two or three years old. Assuming Baby Groot can grow two years in two months, that would make Groot 48 in Infinity Wars. Now, Groot doesn't really seem to experience age or time the way humans do, so these calculations don't really mean much, but judging from the post-credits scene, he'll at least be a teenager.
Will we ever seen him even older? Well, one person certainly pulling for an Adult Groot in Guardians of the Galaxy Vol. 3 is actor Vin Diesel, who described Groot's mysterious aging in an interview with USA Today, saying of GotG 2, "The first Groot we saw was a freshman in college. This Groot is in nursery school." As for the next time we see Groot, Diesel could only speculate, teasing, "Wait 'til we see the full-fledged working-man Groot."
It's looking more and more like we'll be getting Adult Groot down the line, and I, for one, can't wait.