Sometimes when I think of my age I'm pretty sure it is something around 18. It's a bit weird since I'm twenty. I don't know why I keep believing I'm younger. It's not awful to be this "old" or anything. I just forget my age so easily. Like today when I read a story of a 18-years-old girl and for a moment I thought: "she's at my age" and a moment later a realize how silly I was.
In Judaism, 20 is the age at which a person is responsible for his actions to the Heavenly Court (and is thus culpable for his sins). Wikipedia.