Loki: 17 years old. Thor: 18 years old. (in godly years, of course)
They never got along. No; he didn't want to get along. They were simply too different to form an acquaintance. Furthermore, the lack of relationship was not an impediment or any sort of crack in his rather peaceful life; in fact, he was quite content he did not know Thor as well as is normally expected of siblings. He had a feeling that, were his quiet, academic ways to contact and mingle with Thor's careless and flippant lifestyle, his lifespan, if he had one, would be severely shortened.
He never had many friends, but he was fine with it. No friends amounts to no responsibilities or obligations to anyone except oneself. Thor, however, was incredibly popular with various Asgardian folks. Loki would see Thor fooling around with his childhood friends Hogun, Volstagg and Fandral and not understand their happiness of being acquainted.
Moreover, he half-resented Thor for gaining their father's attention so damn easily. Thor was everything he was not, better at everything he was, and in those subjects that Loki was good at and Thor was not, it did not matter because those subjects were trivial in the world of Asgard.
They were like complete strangers. Neither made any measurable difference in the other's life.
But he had his books, which he could dive into and lose himself in fields of blossoming words and worlds, into series of glorious adventures that he could and probably would never experience in reality. The vicarious enjoyment of reading was much better than living real life, because in a book, one can live the lives of many people, feel their happinesses as if they were one's own and experience pain without really receiving wounds or injuries. It was where he was truly happy.
Or so he thought.
Little did he know that the strings of fate were beginning to intertwine his with Thor's and that soon, he will never be able to look at Thor in the same way ever again.