No one is completely certain as to the ‘exact’ science of how page rank is formulated, but it is understood that a number of elements such as age, number of backlinks, and amount of content are used to formulate it.
Years ago, Page Rank was a way for Google to determine a web pages search rankings but now it is just one of 200 ranking factors Google uses to determine a page’s popularity. While still relevant, it is no longer the driving force for determining rank.