Long ago in a book I can no longer access, I spotted a German term for the genre of literature that focuses on aging or elderly characters, including their lives and emotional journeys. I expect that it would be a compound word that ends in "Roman," but I can't say for sure. Could it include the word "Verfallserscheinung," indicating decline or deterioration? This is only a guess on my part.
The book Altwerden ist das Schönste und Dümmste, was einem passieren kann by Reimer Gronemeyer contains the term Geronto-Literatur. This is the only such term that I could find, and the only usage of that term.
In German, the following terms are commonly used in relation to old age and getting older:
Geronto- "of or related to old age or aging", e.g. in Gerontologie
Älterwerden "getting older", e.g. in Die Kunst des Älterwerdens
Altern "getting older", e.g. in Physiologie des Alterns
The two german nouns Altern and Älterwerden are not commonly used to create compounds but rather in phrases. I tried a few searches of compounds and phrases with Literatur and Roman but found nothing. Your best bet would be to try and remember the book title and look at it in the Amazon preview or Google books.
Most likely, the term you're looking for is
Seniorenroman - senior citizen novel
Es gibt ein neues literarisches Unterhaltungsgenre: der lustige Seniorenroman. Sein Entstehen dürfte auf das Jahr 2009 zurückgehen.
There is a new literary entertainment genre: the funny senior citizen novel. Its origin should go back to the year 2009.
An example of this is Jonas Jonasson's "The Hundred-Year-Old Man Who Climbed Out the Window and Disappeared"