This is an example of how myths get started:

Some so called expert buys an aged domain, installs a blog, creates some content, build backlinks, and then goes to every forum they can think of and emails their list saying something like "I rank #1 for XXXX keyword because I have an aged domain. Age is an important factor in ranking".

Then they normally follow it up with some junky ebook that they are selling.

The fact is Domain Age has nothing at all to do with rankings, you can have a 10 year old domain name and it not rank at all. The domain name age myth is spread by all those so called experts.

The length of time a domain is "INDEXED" in Google does have some weight on rankings. The exact weight it has nobody knows but Google, just having a domain indexed for a long period of time is not going to get you the rankings though.

You must have many of the other factors along with the indexed time. Factors such as quality content, proper on-page SEO, proper off-page SEO, social validation, and etc.

The reason why these so called experts spread these myths is because they normally want to sell you something because other wise they would make no money. The reason why people believe these myths are because they want instant gratification. IE: Buying a 5 year old domain is easier then getting a domain indexed for 5 years with quality content.