Editing Yagura:SRD:Character Descriptions (section)

====Partial Rejuvenation ====

More common are effects that cause you to get younger. It's usually beneficial, so you retain bonuses from your former age category but gain the penalties of your new age category. As far as this rule is concerned you cannot be rejuvenated beyond young adult. Your mental age remains equal to your true age.

Unless a number is specified, rejuvenation by one or more categories restores your physical age to halfway between the new category's starting age and the next category's starting age. (A human that was rejuvenated to a young adult's physical age would become 25 by example).