In this video,
Google's Matt Cutts says: "The difference between a domain that’s six
months old verses one year old is really not that big at all."
Going by comments I've seen written about that statement, most folks
seem to think "Oh, it doesn't matter." But, by acknowledging the
difference between a six-month old domain and a one-year old domain is not that big of a difference, he's also admitting it is a ranking factor.
So it's not that big, but he's only talking about a six-month difference
in age. What if the domain is several years older than another? I think
the older the domain, the bigger the difference it makes.
Plus, as with all ranking factors, any one thing isn't likely to make a
significant difference. It's the total of all the factors that matter,
so every little advantage you can get, however small, is still an
advantage. In general, those with the most advantages win the ranking