If I had a kid, I would make sure he/she was the oldest in his/her class for a number reasons: academics, social skills, athletics or other extra curricular activities, maturity...name it. Honestly, I cannot think one negative reason to starting one's kid late, take a year out to home school, repeat a grade in an advanced class, whatever. It certainly would of benefitied a number of students I had who were immature in 9th grade yet came back for their sophomore years much stronger in all areas of their lives.
I am open to opposing thoughts and would love to hear the negatives of starting one's son/daughter a year later than others.