This was one of my most irritating traits when I was younger. For other people, I mean.
As soon as I was talking, I was correcting grammar, we have home videos. I was not a fun person to baby talk around!
When I was in school and we worked in groups, I was always pointing out wrong spellings or things...a little later I started correcting the teacher's writing on the board...what an obnoxious child! But I didn't feel all high and mighty, I just felt the need to tell them, lol. Not really a need, but it just came out.
In secondary school, which was a v. different environment, I quickly learned to shush and not say anything (and even the English teachers made mistakes there!). The few times I did do it, I got sat on (not literally), so even now if someone asks for French grammar help, I'm scared to point out their mistakes. The habit's pretty much broken now, although I'm still a complete pedant when it comes to grammar, only with my family, though. It's a point of interest, since we're none of us native speakers, but if they've had enough they'll definitely tell me!
I can talk about many more general things with my family because I know they'll make allowances, but also I know they won't hesitate to tell me off. Whereas other people will just exchange Looks and not bother again.