"It's about time you grew up."
I expect a lot of people have had that said to them at one point or another - I still get it said to me every now and then, actually - but it kinda hit me quite hard when I was younger. My closest friend lost patience with me one time and told me that after I'd been ranting for about an hour about something I've long since forgotten, and because it was coming from HER, it really got me to thinking about my life, and what affected me, and how I acted, and...well, everything else, I guess. I started to mellow out a bit after that, became a lot more reserved, and stopped letting every little thing other people did get to me. I had a very narrow and rigid view of what being an adult was back then, but I did start looking at my attitude and addressed what were obviously issues that I hadn't even thought of before then.
"Their marks aren't going to get you a job." is another one that impacted my way of thinking when I was younger. I used to be very competitive, and if I didn't get the highest marks out of all my friends, it drove me mad. I HATED losing. But after that was pointed out to me, I eventually stopped caring and started paying a little more attention to my own marks. I still berated myself for not getting higher marks, but it was because I was convinced I could do better, and not in comparison to anyone else. I also took a little more pride in my achievements. Just a little.