It would only make sense for search engines to give more credit to links that have been around for a long time, anyone can go to a place like tnx and buy a bunch of links and monitor the SERPS but few people that are trying to buy their way to the top purchase links for 5 years from a site rather than 2 months.
I think it could go both ways. Think about a link from the front page of Digg (PR8). After a day or so, that link is going to fall down into the deep barely linked pages, as more content gets added it will just get buried deeper and deeper. Same thing with forums, and possibly with a lot of blogs who don't work on deep links. Then again, some blog posts end up with good PR themselves, so it can definitely work either way.