I never heard or thought any parent would do that so their child could have an advantage in sports since playing sports always is categorized by age groups. (no advantage at all)
I completely agree actually.
sports are categorized by age groups.
but are learned in school year groups.
e.g. the child prodigy 5 year old, playing soccer against the 8 year olds in his class will not excel at soccer given his physical disadvantage.
perhaps in an outside of school club he may excel against his own age group, but still in school he will not.
also, it's unlikely that given that he's three years less physically developed that he'd ever play highschool (american) football, or basket ball. because again, given size and development he's always going to be at a physical disadvantage.
I imagine that everyone is nodding in agreement.
now replace that 3 year physical disadvantage with what could only be a few day - 12 month advantage...
that's going to make practically zero difference.