Who is talking in the poem "Once Upon a Time?" Who is addressed and who are "they" that are referred to in the poem?
In my opinion, the speaker in this poem, the person who is doing the talking, is a father. He is speaking to his son. That is, presumably, why the speaker calls the listener "son."
I think that what is going on in this poem is that the father is telling the son how much he (the father) misses his younger days. He is saying that "they" (I believe that he means adults when he says this) are entirely fake. He believes that adults do not say what they mean and do not really have genuine feelings. This is why he wishes he could go back to his youth.