Which month is the "right" month definitely changes by culture, but the premise that being the oldest in your childhood cohort confers an early advantage seems to hold across multiple countries.

If it makes you feel any better, the magnitude of the advantage conferred isn't huge. I'm a September baby, was almost always the youngest in my class, and (at the risk of bragging) I'm objectively successful. I was never great at sports, but being several inches below average is probably the culprit. :)