Why aren't children allowed democracy?

usually under the age of 18 you haven't even taken cles on government, probably barely watch the news, and at younger ages you most likely are still juvenile and immature and so for a child to play a part in what happens in our country would be kind of irresponsible. If it were only one child it wouldn't seem like such a big deal. But there are millions of them in this country. Just imagine if they all wrote in for president "my dog" instead of actually choosing one of the candidates listed. Wouldn't that be interesting.