"Yeah, Castiel's been on Earth for what, ten years? Real amateur hour." It's said in Gabriel's careful blend of derision and fondness, leaning neither direction too strongly. He loves Castiel, but to say his little brother is a bit of an awkward weirdo might be an understatement, and although there's certainly potential there's no real chance of overcoming the experience gap. "Try a few thousand."

And as for mages-- "Sounds a lot like witches in my world. Long life span, and they mostly keep to themselves or each other, sometimes monsters or other supernatural beings too, but unless they're really powerful they usually know better than to draw attention from anything above their pay grade." Hence being loners or sticking with other witches. Unless you're on the level of someone like Rowena, messing around with demons or angels or gods is asking for trouble.