Situations are different for each child, and each child is different, but an example from my life.
My two oldest daughters were two years apart, and our oldest daughter was extremely assertive, an outstanding student and very well socialized. In retrospect, she totally dominated her younger sister.
When it was time for the younger one to enter school, we had a choice of waiting a year or starting her as soon as she was eligible. I feel we made the wrong choice when we started her early. For the first 5 years of her life, she was the youngest child in the house, and was never the leader. Her sister decided what games to play, etc., and dominated her life. When she began school, she was one of the youngest kids in her class, and at that age, the difference between being 5 and being 6 is significant. As a result, she almost never got to be the leader, seldom got he own way. To make matters worse she was in the same school as her straight A sister, and was always being compared to her by the teachers that had them both.
In retrospect, I wish we had started her in school a year later. As one of the oldest kids in the class, she might have had more of a chance to be a leader, and less frustrated by always being dominated by everyone that surrounded her.
Just my opinion.