I am very light on details, it featured some sort of end of the world scenario where you could no longer drink water and had to drink gasoline, and there was a married couple with the woman rapidly aging and the man de-aging and becoming a baby. Any ideas?
Not off the top of my head unfortunately. Any idea of when you read it? Any names of characters? (the strangest little detail sometimes is an important clue in identifying a story)
The story is Something Passed BY, by Robert R. McCammon. I even found it online, at the author’s website:
http://www.robertmccammon.com/fiction/something.html
I’m fairly certain this is what you’re thinking of/looking for. Enjoy! 
That is it, thank you.