DS had just finished kindergarden and was turning 6 when I found out I was pregnant. I had difficult pregnancies, and it was nice that DS wasn't needing much attention anymore.
I think the age of the parents is something to be considered as well. I was 27 when I had my first, and 33 when I had DD. My brother and his wife have 2 boys, they are 18 months and 3 months old. They are in their early 20's though and are up to the challenge. The hardest part for my sister-in-law is that when she is breastfeeding the baby, the older one knows she is busy and he gets into a little mischief.
I am 11 years older than my oldest brother, and 13 years older than my youngest brother. We always joked that I was a built in babysitter. The cool part about the age difference is that my brothers and I have never ever argued. The boys did argue a lot with each other though.