1) In Princess Cookie, she visited the Candy Orphanage. Baby Snaps was still a kid there, and Bubblegum looked 18. It was the past. While Cookie has grown physically, PB still remains the same.
2) In the promo art for Burning Low, Flame Princess seems to very young, like 5 years old. While Peebles in next to the lantern, looking 18 years old.
3) In The Suitor, the oldest suitor said he had been waiting for PB for 300 years. Indicating that she has lived for 300+ years. I think that it is because of Candy Biomass that makes her live.
