At one time I flirted with the YEC point of view as a Christian. What helped me change both (I'm an atheist now) was exposure to more points of view and modest experiments applying biblical teachings personally. After years of twisting logic I could no longer justify the magical thinking. Because none of the experiments were convincing. (Don't forget that God can't be tested! Except when it comes to donating to his chosen leaders.)

And within the Christian communities I saw so much harm from the various biases and non-critical methods it was heart breaking. Sadly it can still be hard to leave when you're taught you're choosing pleasure, hell fire, selfishness, and abandoning absolute truth and love.