As far as I am aware it usually more like natural ability, time and effort that leads to athletic success, not age compared to kindergarten classmates.
I think the point in the book
Outliers about this is that the older, more physically developed kids were considered to have "natural ability." Hence they received better coaching, more opportunities, support, etc. that all played a part in their development into star athletes. Maybe they really did have natural ability, but when Gladwell looks at the birthdates of the players on an elite Canada hockey team he finds a disproportionate representation of January 2-March birthdays (cutoff is Jan 1) on the roster. It makes no sense for Canadian men born in Jan-Mar vs Apr-Dec to have greater "natural ability" in hockey, does it?
Specific dates in each region will differ depending on the cutoff for sports and whether kids are age-normed or grade-normed but the idea is the same. Around here I believe it's age-based at the end of July or August, which should give the fall birthday kids an advantage, and would actually work against the summer birthday jocks as they would be "playing down" or against younger kids when their real competition when it matters is the next grade up. The irony.
I differentiate between redshirting for competitive reasons vs for developmental reasons. In my area, it seems to happen mostly with boys from wealthy families, and there's quite a few of them.
