I gave Aiden a pillow when he was about 16months. I started to notice that he wasnt sleeping well in his cot at night, but, put him in bed with us, even after we got up in the mornings and left him there, he would sleep alot sounder with a pillow. He used to be a bum in the air sleeper, but, give him a pillow, and he's flat out on his back, or, on his side.
So, i prefer flat pillows, so, i gave him one out of my bed, my scent etc, and since then, his cot sleeping his improved heaps. He sleeps 12hours straight now. I introduced it for his day nap, so i could just keep abit of an eye on it, as even at this age, i was still concerned abit giving a pillow, but, he is great with it.