It is up to you whom you call young people. It is contextual, e.g If your grandfather is a happy go lucky kind of person, enjoy every moment of his life happily then you can call him young by heart. If a boy aged 24 and every time look serious and behave like a sad old man then he is young physically but old by his heart.

Thank you

i think young people as the person who is 18 to 28 years old,they are do like not thing result because they are very young.they have many dream but where are don't know stat

If the person is old, but still active and youthful, you could say that they are "young at heart."