I did Tae Kwon Do when I was younger. I didn't keep up too well, but was pretty immature when I was 13-14. These days I'd probably take it more serious.
Lots of people don't take martial arts seriously when they are younger. Unfortunately, I didn't have a childhood, so I took things much more seriously when I was around that age.
I did all sorts of martial arts as a kid... TKD, Karate, you name it. I didn't get "serious" about martial arts until I was in my mid-late 20s, I think.
Actually, I'm still not sure I'm actually serious... :-)