I use SEO quite extensively and that requires that I get a good Page Rank (PR) in a relatively short space of time. I use PPC to test a niche but it is not a long term strategy. I just use it for traffic to test the keyword and conversion rate of my site.
There are a number of things to consider when trying to improve PR and domain age is thought to be one of the compenents. I have personally found that older domains with some traffic history gets a higher PR quicker than fresh ones. I have recently bought an older domain that used to be a directory site (see archive.org to see what your site looked like previously) with 48k backlinks. I can get a PR4+ site with not too much work. I still need to do many of the other things though and domain age (DA) is just one of them. I use Market Samurai for keyword research and it is one of the elements that MS evaluates when it looks at SEO competition. You need to be quick off the mark in the JV business and it is great when you can get a 1st page on Google for a keyword early in the JV campaign.
You also tend to find better names on older ones. some of the ones that I have are 100ksales.com, 1800perks.com (DA 9y+), businessmoneysystem.com, theidiotclub.com (DA 6y+) etc. I have quite a few other great ones in the IM/Money space.
This may not work for everyone but it does for me. I also sell domains that I don't use in a specific niche and reinvest in other. Over time you also build up a nice asset if you create minisites on them optimize, monetize and flip them. Side hobby