Lots of research out there but to briefly summarize part of it:
QUALITY EARLY LEARNING
"Key early childhood research studies demonstrated that high-quality care and education from birth through age 5 results in higher IQ scores, higher school graduation rates and lower crime rates. Young children with high-quality experiences have also been shown to have better vocabulary, language, math and social skills, more positive relationships with classmates, and higher scores on school-readiness assessments."
So think if you hold your kid back at 5 so will have a better opportunity in sports.