young at heart
From Longman Dictionary of Contemporary Englishyoung at heartyoung at heartYOUNGthinking and behaving as if you were young, even though you are old → youngExamples from the Corpusyoung at heart• Arthur's 96, but he's still young at heart.• It's ideal for children aged over five and adults who are young at heart.• Obtain a fifty five Plymouth for the young at heart.