The expression means that middle-aged people are old and young, busy with life, busy running for life, no longer poetic, no longer as emotional and poetic as teenagers.
Middle-aged people only have deep worries, unlike mountains with aura and trees with branches and leaves!
Girls have a crush on the boys they like, but they don't know the boys they like! It is also an enviable youth! The girl's feelings, singing and dancing, the reality without middle-aged people, the pressure of middle-aged people, and the troubles that can't be driven away are as strong as wine!
It's like a poem saying that teenagers don't know the taste of sorrow. In order to give a strong explanation to new words, now that they know the taste of sorrow, they want to say it and have a rest!