Herodotus, the earliest and fullest source for Leonidas, gives his parentage and the unusual succession that brought him to the Spartan throne around 490 BCE ahead of his elder half-brothers' male heirs, but nowhere states a birth year for him.
What would resolve this A Spartan king list or dated inscription naming Leonidas's age or birth year, distinct from the succession narrative Herodotus preserves.
OpenArchaic and Classical Spartan historyHerodotus, Histories