Is birth year more culturally significant in schools there? Here it's not really even a consideration. Ofc the school starting cutoff is usually based on age at the start of the school year in august iirc, not on calendar year, so I had a mix of classmates born in 94 and 95 for instance. Being born in summer made me one of the youngest, and being born in december would place someone in the older half. We only really associated with grade level, and with graduation year (both high school and college). And assuming your sports are through the school system (most are, but not all, and those that aren't are usually more chill) I don't think actual age affects eligibility at all until it's like state and national championship level U16, U18, U23, etc categories.

Idk which specific sport you're concerned with and if it would be done through school or separately, but if we're going with school grade cutoffs then why not just let the dice fall and if they are near the cutoff then keep them home for an extra year? They would be the oldest in their grade, but only by the same margin that the the otherwise oldest kids are/would be.