Groot isn't a kid tho '-' he's into a kid body and mentality only because he sacrificed himself to protect his team. '-' he seems and acts like one but if you put it as it should then he's a disabled adult Well according to the movie
I mean he is in the process of dying like every living thing but it's not a pressing matter so it is a little hard to answer your question, alien or not.
Greetings! I've been studying this earthen language for almost a week now and i believe that i can now carry a decent conversation ~Groot
Special delivery~! We got 564 ton of lumber for you from a certain someone~ -This service was brought to you by Kami Association Request Agency~