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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
Examples from the Web for young at heart
With the collection, Lagerfeld offered a young-at-heart interpretation of his high art.Chanel, Back to the Future
July 2, 2013