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?