This is a hot debate and controversy around the age of domain and how it affects rankings, especially in Google. Regardless of what they argue, it seems like in most occasions, sites that have been around for a long time consistently appears in the top 10.

All factors equal, older sites have had longer time to build content and links. On the contrary, the data shows that domain age alone plays an independent role in how Google ranks websites.

How does one check the age of a domain?

• Use the whois command/service. Either use the whois command from the UNIX operating system or search for one which provides such web service. Easywhois comes to my mind.
• Alexa. Alexa plots rank, page views, and reach. In the overview page, you can get many information, including the site age.

One more tip that could help you see how a site progressed over time since it was first established is the Internet Archive (Way Back Machine).

Although not all versions of the site changes appear on archive.org, at least it provides the most complete and publicly available history of a web site or domain.

A domain/site may have gone through makeovers several times in its lifespan, but still chances are they also rank very nicely in their niche, assuming they do it correctly now.

Should you buy an existing site/domain? Or, should you register an idea as a domain so that you can do something with it in the future?

I would say, yes and no.

Yes, because if this factor becomes more important in the future, certainly you have an unfair advantage over the newcomers.

No, because there are a myriad of other factors that could affect rankings. Perhaps, you don’t want to depend solely on organic search in the future. Just as an example, think about social media and networks. Moreover, algorithms change all the time.

Of course, you want all the advantages you can afford if you are in a very competitive industry. Your choice.