The AI won't declare war on you unless it has good reason (and only AIs with a high Aggression and Expansion rating will consider "your land is very nice" a good enough reason) -- and sometimes, they also want a partner. In my last game as William, I forward settled on Washington, and he confronted me about it next turn. I told him I'd never do it again. Meanwhile, the Huns declared war on Washington. Much later, I forward settled on Bismarck. Again, I promised not to do it again.
The exact very next turn, they both denounced me. The exact very next turn after that, they both declared war. It ended in a stalemate, and we declared peace 10 turns later, and returned to pleasantries.
Just like early war is bad for the player, it's also bad for the AI, so they won't do it unless they have good reason. Hyper-aggressive AIs like Shaka will always DoW you, no question, sometimes even by turn 30 just like people say they used to in Gods and Kings (though I was watching one of MadDjinn's Beyond the Monument videos, and he even pointed out that the Gods and Kings AI was not that aggressive, demonstrating this by being Monty's neighbor and never getting a single DoW). AIs that don't normally DoW will wait until they have good reason, and now that trade routes are a thing, they probably won't DoW if you're their best (or only) trading partner. If you forward settle a warmonger, though, expect a DoW soon. This is how it should work.
However, I HAVE noticed that AIs are far more likely to declare war late-game, when they can afford to move trade routes all over the world, and perhaps you have wonders and great works they want.