Use the phrase "young at heart" to describe an older person who acts or feels like a young person. For example, an elderly person who enjoys dancing, makes funny jokes, or has a lot of energy can be called "young at heart".
"Young at heart" is a positive phrase. If you want to talk about someone old who acts young in a negative way, you can say something like:
He doesn't act his age.
He still acts like a child.