Young at heart
Definition 1 of 1
Pro Tip 1/3Positive Connotation
Expresses a positive attitude towards life, implying vitality and cheerfulness. 
Although she's retired, she’s young at heart, always ready for an adventure.
Pro Tip 2/3Not Literal
Doesn't mean literally young; it's about attitude and energy, not actual age. 
My grandfather may be old, but he's still young at heart.
Pro Tip 3/3Informal Conversations
Mostly used in casual talk among friends and family, not in formal writing or speeches. 
You’re turning 60 soon? Wow, you don’t look it, and you're definitely young at heart!