young at heart
Behaving or thinking in a way that is more youthful than what traditionally accompanies one's actual age. Grandma is still so young at heart—I can't believe she's taking dance lessons at 90 years old! My knees are a bit creaky and my back hurts getting out of bed, but I still feel young at heart and thirst for adventure!
*young at heart
having a youthful spirit no matter what one's age. (*Typically: act ~; be ~; keep someone ~; stay ~.) I am over 70 but I still feel young at heart.
young at heart
Having a youthful outlook, especially in spite of one's age. For example, She loves carnivals and fairs; she's a grandmother but she's young at heart.