The novel explains that IT landed on Earth during an asteroid impact and established itself under the land Derry would be later built on, and initially preyed on indigenous tribes. From that point of view, IT/Pennywise would be billions of years old, but his clown shape wouldn't be that old.
As Steve says, the creature fell to earth as a kind of a meteor during a prehistoric era, which would make it a few million years old.
IT arrived on Earth through an event similar to an asteroid impact, landing in what would later become Derry, Maine. Once there, IT adopted its usual pattern of hibernation that lasted between 27 and 30 years, awakening to kill and eat and then going back to sleep.
In the novel, It's origins are nebulous. He took the form of a clown most frequently, Mr. Bob Gray or Pennywise, but his true form is an ancient eldritch entity from another universe who landed in the town that would become Derry by way of an asteroid and first awoke in 1715.
In both the book and the films, It is an ancient alien creature, older than civilization, and in King's novel, older than our universe. It feasts on the flesh of humans simply because our fears are easy to manifest and they make us taste better.
In the novel, It claims that its true name is Robert "Bob" Gray, but decided to be named “It”. Throughout the book, It is generally referred to as male, usually appearing as Pennywise. The Losers come to believe It may be female after seeing it in the form of a monstrous giant spider that lays eggs.
In their final battle, the Losers' Club was able to kill Pennywise by reducing him to a child-like form as they overcame their fears. In the moments before Pennywise died, his fear is reminiscent of Georgie's own, therefore closing the cyclical nature of Pennywise's murders.
In Stephen King's novel It, Maturin communicates with Bill, the leader of the Losers Club, and helps the Losers Club defeat It by lending them strength. It comes in many shapes and sizes. Most iconically, It takes the form of Pennywise the Clown, whose real name is actually Bob Gray (anticlimactic, honestly).
Also in the 2017 film, Georgie's death is changed. As well as having his right arm bitten off, Georgie is trying to crawl away but he was dragged into the sewers and devoured by Pennywise, with an unnamed older woman and her cat being the sole witnesses of the horrific scene, including before Georgie lost his arm.
However, It must surrender the the laws of whatever shape It takes. Pennywise's strength is also his weakness. For example, if he were to take the shape of a werewolf (as he does in the novel), silver bullets would harm him.
They reach the conclusion that Pennywise has all of the adults under some sort of spell that prohibits them from seeing him. The adults of Derry are shown to be villainous in their own right as well. The adults of Derry are shown to be true villains of the story in the same way as Pennywise.
Stephen King's IT introduced readers to a one-of-a-kind creature that can take any form, the most common one being Pennywise the Dancing Clown, and as menacing as it is, this creature has one big enemy it's truly scared of: Maturin, the turtle.
Though there are some IT characters just as scary as Pennywise, the film's main villain proves to be its deadliest, with a kill count that actually reaches into the tens of thousands. Reddit user u/angelholme estimates that Pennywise actually took the lives of between 12,117 and 18,011 people.
"And the moment you see her performing, you know this is a serious actor." Not only could Gregson bring the convincing warmth, but she turned on the menace as Bev figures out Mrs. Kersh is the daughter of Bob Gray, whose evil clown Pennywise is an incarnation of the shapeshifting creature referred to as "It."
1985: It is finally defeated and physically killed in another Ritual of Chüd by the adult Bill, Richie, Beverly, Eddie, and Ben. 2013: It would've awakened. In the 1990 adaptation, events occur slightly later, with Georgie's disappearance occurring in 1960 and IT's death in 1990.
Pennywise offers Georgie his boat back, but when Georgie attempts to reach out to it, Pennywise grabs Georgie's arm and bites it off, leaving Georgie to bleed to death in the rain.
Humiliated, Pennywise begins to shrink, transforming into an almost Benjamin Button–like, melted clown-baby.
It was said on page 1361 that the Spider being both female and pregnant was a symbolic interperetation. Its not literal, but a representation of whatever equivalent but inconceivable reproductive capability It actually has. Pennywise didn't become pregnant.
Throughout Stephen King's It, Pennywise is referred to as a male, but author Stephen King pulled a fast one on readers in the It book ending by revealing the creature's true form was a pregnant spider, implying that it is, in fact, biologically female.
The spider-clown shrinks as the Losers hurl taunts at It, until it's tiny and weak enough that they pluck out its heart and squash it into nothingness. In the end, they defeat Pennywise by, uh, making him feel really bad about himself.
In fact, the balloons are considered in the fan theory to not only represent the deadlights – but that they also carry the souls of all the children Pennywise has devoured.
The features that we often define individuals by—their skin, their hair, their smile—with Pennywise and the Joker, these features are abnormal and disturbing. There's something twisted about them. They don't feel human.