Astrology is obviously nonsense, but it seems plausible that the season of birth could have a personality effect: the cold dead of winter, with little fresh food vs spring etc, also from the attitudes of surrounding adults.

Of course, different near the equator, and phase-shifted in the southern hemisphere, and... much less pronounced in modern times, with supermarkets, refrigeration, air-conditioning/heating, imported foods, and wider social presence (newspapers, TV, radio, internet, phone) etc.

The socially-constructed age-cohort you go into is huge too. Want better grades and performance at sports? Be months older than your classmates.

The reverse is true, too: Want better performance than people of the same age? Have had more training than them.

I suspect this applies especially to language or logic related tasks.

We will likely never find out, variance seems to be extremely high, while the actual differences are very low.

There are actual studies. Lots of professional athletes were in the older part of their cohort when they were younger. So the effect Terr_ described seems to outweigh the one you describe.