It's a must to register your domain for 3 years or more, if you register your domain for 1 year only google will not value your domain.
it's a the most basic seo for many years now, so most webmaster know it :-)
Google keeps track of the age of domain names and can also tell when ownership changes take place. They consider the age and continued ownership of a domain name to be a good signal of predictability of how the name has been used and will probably be used.
Age and continuity of ownership is more important than duration of registration. It is not in-ordinary, for instance, for many long term domain name owners to register their names on a year to year basis.
Google uses hundreds of signals to judge the content they find on the internet. The domain name is just one of them. Continuity of use of a name is one such signal.
I actually did not know it was such a big deal, I knew length of time actually being owned was big deal, but not length of registration. SEO is not terribly important for my company, but is for my clients.
Rank of a site would normally go down and google would give less value to the sites that have short expiration date as the website may be shut down at anytime. But I don't think there will be significant difference in the indexing rate as long as your website is up.
The relevancy of a website is determined on the basis of quality of backlinks and the frequency of posting new and fresh content. Domain age does not matter in these terms. If you have an old website that is not updated on a regular basis, you might not get good benefits out of it.