Here's the deal on Google-s-SEO ranking (SearchEngineOptimalisation):
The more links the better and the higher the Pagerank of sites linking to you the better. For example, you may only need 1 link from PR 8 page to get PR 7 page and you may need 5 links from a PR7 page to get a PR 7 page, but you will need maybe a 1000 links from PR 5 page to get a PR7 page.
The lower the PR of the site linking to you the lower the PR will be passed on to you.
For example submitexpress has over 45,000 links, so many of them are probably in the PR0 to PR 3 range, but the mass amount of links here is helping us to get to PageRank 7. Longevity definitly plays a roll in natural rankings, as the longer a site has been out there, the more links it will have naturally over time. I've — grapplica — been have been around since the dawn of 2006.
When you look for link partners, you should not even bother with anyone less than a PR 3 page. You can hope that maybe in the future their PR will increase, but why bother, just look for sites with PR 4 or more to start with it. But you can just start with linking everyone you like, hope they'll track your link in Technorati or another tracker and assume they'll be as kind to backlink your pages.