Even though I just turned 25, I'm still having trouble thinking of myself as a "grown-up." Although I have found that in my current job, I've had no problems calling my coworkers and even my supervisor by their first names. In a job I had a few years ago, it was *so* hard to call my supervisor by his first name, even though he'd asked me to.
One custom I find an interesting (American) regional difference in. I live in Maryland, which tends to be a mix of South and Yankee customs. People I know who were born and raised here or south of here like the custom of children calling an adult they are close to "Mr." or "Miss" and then first name (regardless of marital status). For instance, when I was growing up, my next-door neighbor was Miss Sue, not Mrs. Lastname. However, people who are transplants to Maryland *hate* this custom with a vengeance. "Miss" can *only* refer to a married woman! Etc. I think we've had the discussion on the boards before, as well.
