People who think that Princess Bubblegum is 18-21 years old are idiots. It is clearly shown that in The Suitor and The Vault (especially in The Vault!) that PB is over 300 years old...and maybe she was even that Bubblegum in Simon & Marcy. (Just a random guess that she is the bubblegum then)
It's a bit messy, you know. From Too Young, it was where you'd realize that she was 18, since that's the age "she wanted to turn back to." However...recently in Earth & Water, Princess Bubblegum appears to look the same way back when Flame Princess was still a baby, instead of looking relatively younger. This could note that she just MIGHT actually be older than 18 (technically speaking), or that when she was on the verge of getting older than 18, she may have removed some candy from her biomass to shrink down her age to become 18 again. That could note that, in contradiction to her words in Goliad, she could actually "live forever," simply by reducing her candy biomass to age down at least not younger than 18 (because Lemongrab just might take her place in her stead) and then go back to aging "normally" again.
BUT, some fans have speculated the theory that perhaps Princess Bubblegum has a different aging pattern compared to that of an ordinary human being or such. Yet all this is theory that only the creator of Adventure Time would decide is true or not.
Again, there is no concrete way to define her REAL age other than simply assuming she is 18 (as she always is, despite how Finn seems to be getting older as the show progresses, which could give foundation to the theory fans have speculated upon).
That said, it's just my personal reasoning behind the question, and not pure fact. ^^