There is a good deal of red-shirting here in our area and most people are very upfront that it is for athletic reasons. I am not sure which came first though: the red-shirting for social reasons or the realization that having your kid start late would help them excel in sports.

My DS9 (incoming 4th grader) has a mid-August birthday and has been the youngest or near youngest in his grade in public school. DD6 (incoming 1st grader) has a late August birthday (two days ago) and will be the same.

Sports out here goes by either grade or age, depending on the sport.
DS9 is not a really sporty type (tall but beanpole thin and prefers legos and tetherball) so those that go by age are generally good for him but for the fact that, last year in soccer, while he was in third grade, there were some first graders on his team. When he plays basketball, which is by grade, there are kids that are 2 years older than him and are HUGE and often have about 30 pounds on him.

DD6 is sporty. Because she missed the soccer cutoff by 18 days, she will play up in order to play with girls in her grade and skill level. Otherwise, as an incoming first grader who missed the cutoff, she'd be playing with kinder or pre-k kids. In basketball, she was the only girl that played so she played in a boys league. Her first year was on a kinder team-the boys were older but she still excelled. This past summer season, she played in a first-second grade league and it was totally different. Many of the boys she played against were 2-3.5 years older than her (due to her late birthday) and were HUGE. It was a problem.

The interesting thing I see here is that many parents hold their kids back in school but push them several levels higher in athletics (e.g. wanting their machine pitch rookie Little Leaguer to play up two levels or more to AA the next year) just to give them extra experience playing at a high level whether they are ready or not.

Does anyone else see this, too?

We had our DD play up in soccer just to allow her to be with kids in her grade.

I am not passing judgment on this environment out here. It just is what it is.