Because society immediately assumes that young = no experience = bad.
However, I don't consider kids to be bad players. If a 12 year old child (for example) has been playing since he/she was 7, that's still 5 years experience.
Also, I have been told that the younger you are, the faster you learn.
Kids are capable of more than you think.
I don't know what anyone's talking about in this thread.
If a 10 year old kid and a 50 year old adult have the same rating, the 10 year old kid is almost always much better. He's younger, has more energy, and is still improving (while the 50 year old can't expect to improve much).
There are a lot of kids registered in the USCF system with ratings under 1000, and so it makes it seem like kids really suck at chess, but kids who are good can become scary good. I'd almost always rather play an adult than a kid given the choice.
This comes from someone who is straddling the line between kid and adult (17 years old).