The question whether the length of domain registrations had an influence on search engine rankings or not has been around for a while. But so far none of the big search engine companies has given any firm advice on this subject.
Today, however, I stumbled upon a post on Search Engine Land saying that domain registrations probably have a small influence on rankings in search engines, but that other factors (quality of content, amount of links, etc.) have a lot more weight. This isn't surprising to me, but I still believe that the age of a domain as well as its future expiration date do affect search rankings, even if only to a small extent.
Barry Schwartz from Search Engine Roundtable quotes John Mueller, a Google employee:
A bunch of TLDs do not publish expiration dates — how could we compare domains with expiration dates to domains without that information? It seems that would be pretty hard, and likely not worth the trouble. Even when we do have that data, what would it tell us when comparing sites that are otherwise equivalent? A year (the minimum duration, as far as I know) is pretty long in internet-time :-).
But another Google employee and SEO expert, Matt Cutts, gave less clear advice, as quoted from Search Engine Land:
My short answer is not to worry very much about that [the number of years a domain is registered], not very much at all.
In the same post, Search Engine Land says that when they had an interview with a Yahoo employee some time ago, that employee said that domain registration lengths did matter. So while most search industry insiders say that domain registrations have only a small influence on search engine optimization, none of them clearly states that they have no influence at all.
My interpretation is this: Concentrate on creating high-quality websites with unique content providing valuable information to visitors and that attract lots of incoming links. But if you ever get the chance to purchase an aged domain, go for it as it will most likely improve your chances of getting a better ranking in search engines. Also renew your domains well in advance if you already know you're going to own them for a long time, because this might have a positive effect on your search engine position as well. If nothing else, you won't have to worry about domain expirations for a couple of years anymore.