I would use two words to describe the mood in this poem. I would say that it is wry and wistful.
In the poem, the speaker is sort of sadly amused at himself. He looks back at his attitudes back when he was one and twenty and thinks about how silly he was back in those days. He thinks it is pretty funny that he had those sorts of attitudes. He also, I think, sort of feels that it would have been nicer if he hadn't been so immature.
You might also say that the last line of the poem changes the mood. It sort of makes it more of a funny poem. Once we see the last line, we cannot help but laugh at the speaker who is looking back on his youthful foolishness even though he is only one year older.
So I think you can use either answer -- the mood of the speaker is wistful and wry, but the reader's mood once we read the last line is one of tolerant amusement at the foolishness of youth.