difficult child 1 was difficult from the moment I met him. He was 3 1/2. We blamed it on his exposure to rough lifestyles and abuse for many years. Around age 7, I started asking questions, and at 14 I started demanding answers - this wasn't aftershock from what he saw as a small boy.
difficult child 2, well, I asked my ob about Ritalin when I was 5 months pregnant. He was an odd, tho very easy, baby, but it was when he started walking at 8 months that I first called my mom, bawling because he was out of control. He had no attachment and no language at all going in or coming out of him. It was another 6 months before my first trip to the pediatrician voicing my concerns to her (which were discarded, by the way). And another 6 months before the second trip (Which was also discarded, and after which, I discarded her and started seeking help).
easy child 1 was a colicky baby, screamed for hours on end every day (second shift, 3-11pm). We laid him on the floor over a pillow and literally beat on him. As he grew, he was a hard-headed little lad, but he was still "normal".