"Freshness is one of the more recent factors that Google has added to their algorithm in the last couple of years.
In fact, the Caffeine web indexing system was meant to help them find and identify fresh content among the massive amount of stuff that gets published every day…an impact that affected 6-10% of searches".
Here 4 Content Freshness Tips:
1) Creation of new pages will impact freshness;
2) Document changes affect fresh content;
3) Time on page;
4) New links to page.
Via Antonino Militello