Wow. I took it to mean something a parent might say to a boy who wants to drive the car, bench press weights or maybe drink vodka or something. Meaning that this is something he can look forward to doing when he is a grown man.
I mean, as a woman, I didn't read any insult into learning how to say "I am a girl." It could be a statement of fact, not meant as a put-down.
When you’re talking to one person and don’t require a formal verb form. For the full conjugation of essere, see https://www.verbi-italiani.info/en/conjugation/96-essere.html .