So am i a grown up yet?

I'm 19 and a female, from Britain. So, the legal age for everything is 18, and ever since I hit the big 18 my older sister uses the "you're a grown up now" line. But, I'm only at college, can't even find a part time job and I don't really do anything or worry. I feel like I should but I just don't feel like I've matured at all since 15. What's your opinions on growing up? When I was 17 one minute then 18 the next, did I enter a whole new era of my life because I aged a few hours?