Age is often important, however my best site gets in the top 20 for whatever I post in my blog, yet only a year or so old. I guess that means trust is important, so if you buy an old site with no trust or one that has been misused, then it will not benefit from its age.
In my experience with domain age, the age plays a major role. This is an effective way for the search engines to rule out that you are not a website that is looking to get listed and spam people. Age is a fine wine, it shows you have been around awhile and are sticking around. I had a client never had a website, it took 6 months for them just to get listed for one keyword then I had a client that had a 9 year old domain, I redid the site and within 5 days was listed for over 10 keywords on the search engines.
I would like everyone to have a look at the following list from SEOMOZ: Search engine ranking factors. As most of you probably already know, SEOMOZ is a great source for information about SEO and PPC campaigns.