when is is you are officially a grown-up?

obviously I am an adult at 42 but I still have trouble with the concept of being a grown-up....My SIL made reference of herself as Mrs (insert her surname here) and my first thought was what does my mother have to do with this conversation? Of course similarly any time anyone calls me Mrs (insert my surname here) I immediately think no no you are looking for my MIL. When I was a kid my parents always seemed to have everything under control yet most days I feel like I am flying by the seat of my pants so when is the magic moment that I get to sit at the grown-ups table?