the narrator in housman's " loveliest of trees" the Cherry now mentions his age while admiring cherry trees. what theme does the narrator express by doing this

I would think that it reflects upon how he is aging in a sense. You could say that the theme of what he doing is growth. He says his age, and then admires the cherry trees. Not sure if your teacher wants a full explanation, but I would go with growth of him as a person/age or just growth.