"An Old Story" was a short story from the book Still More Tales to Give You Goosebumps.
Tom and Jon spend a lot of time home alone because their parents work so much. One day, their old Aunt Dahlia shows up to take care of them and starts cooking dishes loaded with prunes. Tom finally notices that his brother has a big bald spot and that he has thinning gray hair. It turns out that Dahlia is a sorceress and the prunes were enchanted, so the fruits cause the two boys to age rapidly. Dahlia brings over some of her elderly friends to play bridge, all of whom immediately start fighting over the two.
Tom goes to get some milk, gets offered the senior discount, and gets back in time to hear the senior ladies talking about marrying them and Delilah demanding her fee. The two adolescent brothers then realize that they are being magically aged into elderly gentlemen so that their aunt can sell them to her friends as marital partners. Tom tracks down a jar of wrinkle cream, puts it on, and instantly reverts back to a teen, and Jon does the same thing.
Dahlia chases them through the house until Tom hits her with a pitcher of prune juice, which makes her age so fast that she turns into dust. Their parents later realize that they each thought she was the aunt of the other and actually had no relation to them. Tom heads to school the next day and sees his crush eating lots of prunes that her aunt packed for her lunch. He warns her to toss them and then shares his sandwich with her.