I'm 26, and I'm still a big kid. I was designing go-karts and their drivetrain setups on paper in school when I was 10, and also did some electronics, and started soldering when I was 12.
I was living a very hard family life at the time, so I didn't get to do much with go-karts, even though I really wanted to at the time, and when I became an adult, I decided to persue that and build my 2 bike trailers, and pursue go-karts again.
I admire the intelligence and freedom you youngsters have with building your karts. I'm still only 14 at heart myself, and age is just a number anyway, I'll never grow old!
Doing DIY stuff when you are young is the best way to expand and grow your mind at a young age, which makes you more competent as an adult!