1) In Princess Cookie, she visited the 糖果 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 下一个 to the lantern, looking 18 years old.
3) In The Suitor, the oldest suitor 说 he had been waiting for PB for 300 years. Indicating that she has lived for 300+ years. I think that it is because of 糖果 Biomass that makes her live.