when people got smart.
No, that was when Atheism started :P
Probably for as long as there have been people. Agnosticism is an idea, and a common one a that, people have thought like this for a very long time. There would have always been some people with doubt.
And this question was answered when you asked who started it. It's not a religion. It's simply a belief. As long as there has been thought about a higher power, there has been agnosticism.
could not of worded it any better myself ty 1_~
haha yeah true:)