Ohio Revised Code Title XXIX. Crimes Procedure § 2929.023

A person charged with aggravated murder and one or more specifications of an aggravating circumstance may, at trial, raise the matter of his age at the time of the alleged commission of the offense and may present evidence at trial that he was not eighteen years of age or older at the time of the alleged commission of the offense. The burdens of raising the matter of age, and of going forward with the evidence relating to the matter of age, are upon the defendant. After a defendant has raised the matter of age at trial, the prosecution shall have the burden of proving, by proof beyond a reasonable doubt, that the defendant was eighteen years of age or older at the time of the alleged commission of the offense.