Is there a phrase or word for someone who seems doesn't care about many things because they have had so much experience or have seen a lot. I'm trying to describe a character in a book. Ex. "You are a demigod, walking with a god, on your way to camp to meet others like you. Doesn't any of this come as a surprise to you?" "Eh...I've seen a bunch of weird stuff."
Jaded. Bored or lacking enthusiasm, typically after having had too much of something. https://en.oxforddictionaries.com/definition/jaded
Or, as Zan700 suggested:
World-weary. Feeling or indicating feelings of weariness, boredom, or cynicism as a result of long experience of life.
Worldly: having a lot of practical experience and knowledge about life and the world. MW
Sophisticated: having an understanding of the world and its ways, so that you are not easily fooled . . . Cambridge
Blasé: apathetic to pleasure or excitement as a result of excessive indulgence or enjoyment : world-weary. MW