My older baseball-playing son entered college at age 17 (Fall birthday). I would say the single biggest issue going back to 5th/6th/7th grade was that socially he gravitated towards the kids a grade lower. Academically and athletically he was fine, but social maturity was probably a little behind.
IMO...looking back...the time to have adjusted this was in 4th/5th grade. I would not have done it for athletic reasons. Academic? Yes. Social? Yes. Athletic? No...not even if I thought that was the only way for him to succeed athletically.
On the other hand, he had a college teammate (at Stanford) who was held back in 6th/7th grade (going off memory here) and that teammate did great in every aspect...made it to the big leagues within about a year of being drafted (after only 2 years of college ball).
Each kid is different. There is no way for any of us to properly guide your situation. If you're really seriously considering it, I would talk to some teachers and try and figure out the social and academic ramifications.