Is it possible to become less smart over time?

I’ve noticed that as I’ve gotten older, I’ve had less time to do things I love—reading for pleasure, drawing, making music, etc. Now I’m a senior in high school, and school takes up most of my time. As a result, I get pretty good grades.

Aside from the grades though, I feel like I’ve been dumbed down over the years, like I’m not as bright as I was. Maybe it’s a lack of time, or other issues in my life becoming prominent. Nevertheless, I want to change. I’ll be going to college in the fall, and I want to be as intellectually prepared as possible.

Is this an intellectual issue, or an emotional one?
Is this normal, or even possible?