To act one’s age - behave more maturely Image shared under CC0 Jane’s grown-up daughter now acts her age. She controls her emotions maturely even if she is upset or angry.
To be young at heart – having a youthful spirit no matter what one's age Image shared under CC0 Our parents are in their seventies now, but they are still young at heart.
To age out - (for an adult) to grow (mentally or in years) out of certain behavior or out of a group or classification that is based on age Image by Stephen Bowler shared under CC BY John loves watching cartoons even though he’s aged out of the target audience.
Golden age - a period of or defined by outstanding excellence, quality, prosperity, or achievement Image shared under CC0 He grew up in golden age of rock, when nearly every young guy in his neighbourhood played in a band.
Over the hill - no longer able to do something at an acceptable level because of age Image shared under CC0 My grandpa proves that you are never over the hill to be romantic.