In the season five episode, "Simon & Marcy," we see:
A large, self-aware pile of pink goo (bubblegum) who gives Simon some soup to help cure Marcy's cold. I can only assume this is an early-stage version of Princess Bubblegum (much like how Simon with the crown is an early-stage iteration of The Ice King) because this show.
This would indicate that Princess Bubblegum is way way older than Finn and Jake. Do we have any indications that she remembers the Mushroom Wars?
There are no such indications, and it's very ambiguous as to whether
that pink puddle of goo is Peebles at; it seems likely that they are related in some way (an ancestor? The primordial syrup from which the Candy creatures eventually evolved?), but that is speculation still.
PB has made note of having a great-uncle Gumbald (who chopped down a large number of taffy trees) in S02E18, Susan Strong. Of course, it's entirely possible that he isn't really a great-uncle -- he could just be someone she likes, or something -- but this throws some more doubt on the identity of
the pink puddle.
