Subdomains are basically a subset of the main domain.
If we take blog.therankmachine.com,
Then “.com” is a TLD or a first-level domain,
“therankmachine.com” is a second-level-domain and
“blog.therankmachine.com” becomes a third-level domain.
Webmasters occasionally refer to subdomains as third-level domains.
Example: If your domain is https://www.example.com
Then your subdomain will be https://blog.example.com
“blog” here is a subdomain of “example.com”. At the same time, “example.com” is the root domain or “blog”
Suppose you have a global website, which delivers product to most of the nation. But to manage the website on just a single domain will be quite hectic. So, the solution for this can be creating a subdomain for each country, that way you can manage the subdomains of each country easily.
Domain Registration Length means for the number of year’s, the domain renewal amount paid in advance to the Registrar, You can pay for at least 1 year & maximum 10 years. For every domain bought, you have to pay a yearly renewal amount.
Webmaster says Search Engine gives more preference to the domains which are registered for a long time. Because domains which are bought for spamming web are generally registered for not more than a year.
What does Google say?
Officially, Google’s Matt Cutts has declined by saying: To the best of my knowledge, no search engine has ever confirmed that they use length-of-registration as a factor in scoring(for SEO). If a company is asserting that as a fact, that would be troubling.
If you don’t have time to watch the video, Matt says: “My short answer is not to worry about that very much. Not very much at all, in fact. Danny Sullivan had asked about this recently because there were some registrars that were sending around emails…that said: “Did you know that Google gives you a bonus in ranking if you register your site for 3 or more years?”. Just to clarify, that’s not based on anything that we’ve said. My short answer is make great content, don’t worry nearly as much about how many years your domain is registered for. Just because someone is sending you an email that says “Google does” or even may use this in ranking, it doesn’t mean that you should automatically take them at face-value. What makes a really big difference is the quality of your site, and the sort of links that you have pointing to you, not “Is my site registered for 3 or 4 years, instead of 1 or 2 years?”
The simple answer to the Question is: Yes, Domain Age is a Ranking Factor & Google cares about your indexed age.
As Matt Cutts stated:
“Domain age does play a role, but a small role & it’s not a very strong ranking factor as compared to others.
The main Problem associated with buying Old Domains is you have to find out with what it was associated before? Because domains that get Expired may have to be used before for the purposes of Gambling, Spam, Illegal Content, Adult Websites etc.
So, if Search Engine finds out the Profile History of the expired domains, then your Website will never Rank.
Here’s an Amazing Thing:
We have done a full research & have given practical examples for the Topic which will clear all your doubts.