young at heart 人老心不老；内心年轻；年轻的心

【例句】
My granny is a big fan of rock and roll music – she is 75 and still young at heart.
我的奶奶是个摇滚音乐迷，她虽然已经75岁了，但仍人老心不老。

Many of us were just kids then, we're still young at heart now.
我们大多数人当时还只是孩子，我们的心态到现在依然年轻。