In the season three episode, "Too Young," Princess Bubblegum has de-aged herself to being 13 again. In her de-aged form, she seems to have all her memories and knowledge from her 18-year-old self, but she is the same age as Finn and displays similar levels of maturity and, well, behavior.
However, at the end of the episode, when she is re-aged to her 18-year-old self, Finn nervously suggests they kiss again and Peebles says something to the effect of, "Oh Finn, that was like five years ago."
What explains this time difference? Was she making a joke about their age difference? Or does she remember that event taking place five years ago because (in a literal sense) her body was five years younger when it happened? If so, then why does her younger-aged version of herself retain her older self's memories?