Some people think that Loki is in fact the older brother: They are incorrect. They say that he is older because in The Dark World he says the throne is his birthright. The throne is his birthright, but he is not the eldest. He is second in line. My proof:
He has every single trait of a younger sibling known to man.
Thor, at some point, calls him 'younger brother' or 'little brother'.
My proof that he is less then a year younger then Thor, they are close, but Loki is still younger:
"There was this one time when we were children, he transformed himself into a snake, because he knows I love snakes. So I went to pick up the snake, and he was like 'ergh! It's me'. And then he stabbed me. We were 8, at the time."
-Thor