I have a fear of growing up?

I'm turning 16 this week, but I don't feel like I usually do. All my past birthdays, I have had countdowns for and feel really excited when they're about to come. This year, I'm pretty chill about it, and don't seem as excited. I think from 15 to 16 is a really big difference. I'm taking my first SATs tomorrow, and I feel like I'm growing up so fast. Only two more years of high school and I'm off to college. I'm starting to feel like I'm getting really old. I'm scared of growing up. I know it's inevitable, but it's just so scary to think about it. :[ Does/did anyone else feel this way? What did you tell yourself?