I am glad to see you being preemptive in figuring out just what might be on a test for the Super Sad True Love Story (which is a PERFECT novel for teens to read today, by the way). I want to give you the essay question you ask for and follow it up with a full explanation of the theme of obsession in the novel.
I would suggest the following essay question:
How do Lenny and Eunice exhibit their obsession with youth at the beginning of the novel, and how do Lenny and Eunice overcome that obsession through different aspects of their family relationships?
Now, through my analysis of theme, you can find the answer as well! There is an obsession with youth and youth culture in this book. It is absolutely manifested in the characters of both Eunice and Lenny. In presenting his futuristic dystopia, Shteyngart ends up presenting a satire of the current American obsession with youth culture. Note the similarity of the social network called "GlobalTeens" with the social networks of today: Facebook, Twitter, MySpace, and even Pinterest. Lenny is almost forty years old and is pitied (and even despised) by his workmates. Although Eunice and Lenny are in a relationship, they eventually break up.
I feel safe with him because he is so not my ideal and I feel like I can be myself because I'm not in love with him.
Eunice ends up dating Joshie because he is a provider. Further, Lenny discovers the importance of his parents in his own growing-up experience.
Remember this... develop a sense of nostalgia for something, or you'll never figure out what's important.