Gwnedolyn Brooks seems to choose the inhabit the persona of a group of young men who believe that they are "real cool" because alll they do is drink, dance and play pool and have not achieved anything in life in order to, in the final line, turn the tables on such individuals and profoundly challenge them about their lives and what they are doing with them. Note how this achieved through the final shocking line: "We / Die soon." This ending represents a distinct shift in tone from the rest of the poem, where the slick rhyme and short lines seem to create a kind of "cool" tone where the young group of males revel in their lives and how they spend their time.
The final line of the poem therefore reminds them of the kind of lives that not having finished school and devoting themselves to pool and drink will give them. At best they will "enjoy" a life that is severely curtailed and will end in an early death. We could also see the final line as refering to a kind of spiritual death as the young men find that their ability to appreciate anything good in this world dies. The poem is therefore meant to challenge such individuals and to shake them out of their complacency and make them realise that they need to change their life. There is nothing "cool" in an early death, after all.