The open ended parentheses leads us back to the beginning where youth just keeps on growing old...age sticks. It is a never ending cycle and a beautiful poem.

what is this poem really about

I find this to be one of cummings' best poems. It's all about the conflict between adults and/or authority who set up rules and young people who rebel. Notice that everything that "old age" says is IN parentheses (signifying their stifling control) and everything that "youth" says is OUTSIDE of parenthese (signifying freedom). Much if what "old age" says begins with a capital (signifying authority) and everything "youth" says is in lower case. The split words give cummings a double meaning. For example, breaking up "Mustn't" in the fourth stanza into Must/ n't signifies both those things adults say we MUST do and those things they say we MUSTN'T do. Better yet, the last two lines break up the phrase "growing old" into gr/owing old signifying that not only do young people eventually grow older, they also OWE something to their elders who have set boundaries to keep them safe, help them mature, etc. This is cumming's point--youth may rebel against the restrictions "put up" by their elders (who were once rebellious youths themselves remember) but they are who they are in part because of what their elders have taught them. And they too will grow older, only to set rules and restrictions for their own children someday. How do we know this? Notice that that the last word and the first word of the poem is "old." Just like the cycle of life, you can go from the bottom of the poem right back into the beginning of the poem to start the cycle all over again. And notice too that if you do this, the "old" of the youth at the end of the poem, which is outside of parentheses becomes the "old" of old age at the top of the poem which is in parentheses. The rebellious youth have grown up to establish restrictions of their own. This is truly a remarkably complex poem by cummings though on the surface it seems so simplistic.