Honestly? 
After a while, everything gets just...old. YOU start to feel old. No matter how old you are, when you are with people of your age you feel like you're watching CHILDREN. It doesn't even matter that you should be one of them, that you should be a CHILD as well.... 
You just feel OLD. 
After a while you feel like you know everything your 'friends' are going through. And when you tell the this, they don't understand. They are just CHILDREN, unlike you. You feel like you know everything, even though you KNOW that you don't. 
It doesn't matter to you anymore...
because...
YOU FEEL OLD.