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1.
This phrase is different then child like. Young at heart is like still playful still likes to do fun stuff and not have a care in the world. Childlike is just annoying.
She is young at heart.
by 000000001111111 November 04, 2008
 
2.
Adjective: A fun personality.
Dude A: My buddy makes cartoons for a living.

Dude B: Yeah?

Dude A: Yup, he has two kids, but he's still young at heart.
by Peacestar August 14, 2011