I remember a short story in which multiple generations of people being crammed into a small apartment in a city. The magical drink that either reduced or completely stopped aging was something like a soda that came in a six pack. The story focused on visual aspects of the drink at some point. The younger family members complained that their elders were living too long and apparently decided to water down the soda, I think that conclusion was ultimately left up to the reader. Many thanks for any help tracking down this story, might be one of Kurt Vonnegut's perhaps?
The Schwartz family, headed by 172-year-old Harold ("Gramps"), lives in a three-room apartment on the 76th floor of "Building 257 of Alden Village, New York City, Connecticut". Gramps' grandson Louis, his wife Emerald, and 20 other descendants are crowded into the space, perpetually jockeying for Gramps' favor. <...> Lou then catches his great-grandnephew, newly wed Mortimer, diluting Gramps' anti-gerasone in the bathroom.