Is there a perfect age gap?

I know it's a question I asked a lot when contemplating having another child. I remember after having our first daughter, one of the first sentences out of my mouth was "she's going to be an only child" and then down the track it turned to "when she's 5 we will start trying for another". Don't ask me how it happened but all of a sudden babies were all around me and it felt like our daughter was getting ready to go to school; I just had this unbelievable need or want even to add another member to our family.