There is a mythical creature that is said to live in the Americas called Bigfoot and also known as Sasquatch. This is the same creature as the one made by Victor Frankenstein because the creature told Victor his plan for when he would be done with him.
“I will go to the vast wilds of South America. My food is not that of man; I do not destroy the lamb and the kid to glut my appetite; acorns and berries afford me sufficient nourishment.” He said this on page 176 of the novel. At the end of the story, the creature says it will make a pyre and burn itself to death but we have no way of knowing if the creature really does this as he “was soon borne away by the waves and lost in darkness and distance.” Since the creature struggles to stay alive for the whole story, we have no reason to think it will actually kill itself but instead it most probably went south to the New World (the Americas). As for it surviving all these years, the answer is simple: the creature can’t die from old age because it is already dead.
Bigfoot is said to be around 8 feet tall, have a pronounced brow ridge, have black hair and a strong unpleasant smell. It is also said that Bigfoot is omnivorous and nocturnal. All of these things are true for Frankenstein’s creature as well. This is why Frankenstein’s creature is still alive and is no other than the mythical Bigfoot.