When I went to a dressmaker for my cotillion dress, the dressmaker first said "This isn't a wedding dress, is it?" and I said "no." She replied "Good - how old are you? 13, 14?" I was 18.
I always feel like I look young in comparison to lots of girls - like they have a more sophistocated look. But then I realize that they're willing to sacrifice their comfort to wear black pants and one-shoulder tops to the grocery store on Saturday night, when I can be happy in jeans and a dressy sweater
I started looking older when I cut my hair to above shoulder length, got rid of my straight cut bangs, and got highlights. I also find that wearing "bigger" earrings helps too. Not giant doorknockers, but I have a pair of CZ studs that are about 1 ct each. Not huge, but big enough to look like something a college student would wear.