"Basically it says that registering a domain for 5 years or longer will cause search engines to perceive the domain to be more legitimate and therefore to rank it more highly."
That's a rumor started by a
Google patent dealing with Historical Data that mentions domain age, link age, page age, how frequent a page's content is updated, and a whole bunch of other age factors.
That document gives me a headache. Here's
a summary in English.
Most spammers don't register sites for 5 years, so that's another reason why registeration duration may be a quality signal.
Personally, I just pay every year.