You aren't too old.
I coach a girl who is turning 15 this week and will be competing L5, as well as a girl who is 16 and competing L5 for the second time. I also coach a bunch of L5s who will be freshmen in high school. They were 14 and in 8th grade when they competed L4.
Were they older than most competitors? Yes.
Were they the oldest? No.
Did they have fun, get in shape, and enjoy competing? Yes. That is what matters.
Quick story: When I was 15 I started gymnastics again in an intermediate class. Though the class was for 9-15 year olds, I was the only one over 12. The younger girls loved me, because they thought it was so cool that I wanted to do gymnastics too. As a college student, I worked out with my college club team. Though my skills were roughly L5, even 10 year old L10 girls who worked out in the gym would cheer me on. People who love the sport want to see other people love the sport. Age doesn't matter.
Now, you probably won't go to the Olympics or get a college scholarship, but you can enjoy competing, learning, and having fun. You might consider club gymnastics in college if you are interested.