Marceline/Analysis of Marceline's origin

Facts
"Memory of a Memory" shows Marceline aging over time from a young girl in a period, presumably after the Mushroom War, to slightly older (but not as old as current Marceline) during the incident where her father ate her fries to something close to her current depiction.

Young Marceline has the same pale skin, pointy ears and fangs that she has as a teenager or adult. However, little Marceline hops through trees instead of floating over them, appears outside in the daytime without protection, and has no bite mark on her neck. The older Marceline during the fry incident also does not have a bite mark. The mark only appears on her neck when her memories of Ash begin.

It is unclear when Marceline became a vampire or how Marceline is able to age over time since typical depictions of vampires have them remain frozen at the age they are bitten and are only able to reverse the appearance of time to give the appearance of looking younger instead of progressing in age to become older in appearance. Andy Ristaino has stated she is "kinda a shape shifter" though, so this could explain her growth. According to Natasha Allegri, she was not bitten when she was 16 or 17.

According to Pendleton Ward, Marceline was probably a vampire before the apocalypse.

Source of Marceline's physical traits
Marceline's pale bluish skin, ears and fangs are all similar to those of her father and could have been genetically inherited from him. According to writer Adam Muto, Marceline's father is more of a demonic creature than a vampire.

It could be possible that her status as Vampire Queen means she has more powers than standard vampires (like her ability to change into various monster forms including bats, wolves and tentacle beasts), so she could possibly have become a vampire in the first memory and used special powers to age more, though how she defeated the Vampire King at such a young age is questionable.

Fries
In the "fries" memory, both teenage Marceline wanted to eat fries, something that vampires presumably would not be interested in, since they mainly consume blood and shades of red, but it could also have been for the ketchup.

In "Henchman," Marceline says that she "kinda likes" the pain she feels when she's in sunlight because it reminds her of when she would scrape her knees when she was a kid and her mom would "patch her up." This both confirms she actually has/had a mother and points out there might be a possiblity that she feels love or some type affection towards her mother.

There is a possibility that Marceline's mother was a vampire, and the traditional vampire traits like pale skin and fangs were passed from her to Marceline. However, this would not explain why Marceline currently has a bite mark on her neck, but not have one as a child. It could be possible that Marceline gained her immortality only after getting bitten after her teen years, but was technically a Vampire all her life.

Marceline's Age
Marceline has commented several times that she is "over a thousand years old", which makes one think based upon Memory of a Memory that the Mushroom War occurred a thousand years ago. However, one could argue that Marceline could be fibbing about her true age based upon the evidence regarding the Ice King and his origin as revealed in Holly Jolly Secrets Part Two. In the episode, the ice wizard is revealed to have been a human living in civilization before the discovery of the ice crown, which mutated him into his current supernatural state. As previously revealed by the creators, the Ice King is at least 600 years in age, which means that humans and civilization were in existence at least 600 years prior to the start of the series, to the point that national cultures and boundaries still existed (with him mentioning he found the crown in "Scandinavia", which is an actual location on modern Earth). Yet if Marceline and her memory were to be believed based upon what she said, then the Mushroom War was a thousand years ago, with a four-hundred year gap (at least) between her childhood memory and when the future Ice King finds the crown.In "Marceline`s Closet" said she it took her 500 years to make her Journal.

From this, a few possibilities seem to exist regarding Marceline's age and, likewise, the time of the Mushroom War and it's aftermath:
 * The first is that the War did occur a thousand years prior and that the Ice King is older than the 600 previously mentioned, closer to Marceline's age which, as stated, is at least 1000. This would most likely mean he found the crown before Marceline's first flashback, whereas he lived in civilization pre-war and Marceline was first seen during/after the war.
 * Another possibility is that Marceline is lying about her age and is closer in age to the Ice King's 600. As known, Marceline fibs about things all the time to make herself greater than she is, such as riding on giant goldfish in the middle of the daytime. Thus, she may be lying about her age to make herself look greater and is closer to the Ice King's actual age, thus making the Mushroom War occur 600 years ago instead of a thousand.
 * There are other avenues of thought on this subject. Marceline's first flashback depicts a war torn landscape. However, we cannot be sure this is the mushroom war. You'll notice the area looks nothing like the current one seen in Adventure Time. We are led to believe it was the mushroom war which caused the world to resemble what it's depicted as in Adventure Time. Therefore the war seen in her first memory could either be a different war, or perhaps an earlier stage of/precursor to the mushroom war. Marceline may have been a normal aging child during this particular conflict. Keep in mind we're told that she was probably a vampire prior to the mushroom war. We see her as a teenager when she walks in on her father eating her fries. She does not have bite marks. She also appears younger than her present state. The scene is set in a torn up diner. This may still be the set during the same war seen in her first memory. From this we can look at a few things. The mushroom war itself may not have happened for hundreds of years after Marceline's first memory. In that time she could have been bitten, and therefore immortalised. She may then have been a 400 year old vampire at the time of the ice kings crown discovery, and the assumed conclusion of the mushroom war. This would make her truthful in her claims of being 1000 years old. It would also allow the ice king to be 600. In conclusion the mushroom war appears to be more likely around 600 years old, the same age as the ice king.
 * Another possibility that allows for both Marceline's and the Ice King's ages to be correct is that Marceline's mother was a vampire, causing her to be born as a vampire. Given that there is no evidence that being bitten is what caused Marceline to become a vampire, this possiblity is valid. If we assume that vampires age very slowly, she could still be "a child" by the time of the Mushroom War. It could be possible that the memory Finn and Jake viewed of Marceline as a child shows her when she is 400 years old. If she currently appears to be 18 but is 1000, some simple math (18/1000 = x/400; x= 7.2) shows that if we assume a linear change in what age she appears to be over time, when she is 400 she would appear to be about 7.2, which she indeed does. It is possible that being bitten did not turn her into a vampire, but simply granted her her vampire powers. This means she could have been bitten at any time before she moved in with Ash, but after her dad ate her fries. This possibility suggests that the mushroom war occured 600 years ago.

As of this time, none of these theories can be taken as the true one, thus Marceline's age remains in question outside her own personal words of being "over 1000".