We all know that PB is 18, right? Well, she's just like Marceline. She looks young but is actually older than she looks. Proof?
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.