User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-11492600-20130702221941/@comment-3146930-20130708030632

That always confused me. The gods made him immortal as long as he is needed. But he is a centaur, a monster, so even if he died he would reform. Also, as far as I know centaurs don't really age. In The Sea of Monsters, he talks about his relatives like he has known them for centuries. You could say that his immortality prevents him from being killed, but he has been shown to be injured a few times in the series so he could die.