As per my experience and knowledge, I would say that Page Rank is decided considering three factors:
1. Backlinks: There are many factors that Google will consider when analyzing a backlink, that is Backlink provided websites pagerank, Follow attribute, etc.
2. Website Traffic: Website visitors play a major role in deciding the
PR of a website. If your website is getting on average 1000 visits per month, then surely you will get a
PR for your website.
3. Domain age: Domain age too plays a vital role in
PR.