Hiya Novandak,
My best advice is that you should only treat "Domain Age" as an indicator... First of all domain age is only relevant from the moment G and other search engines index content... In otherwords if you buy a domain name and you never host it etc... then it does not matter how many years later you keep it as it has no history! Therefore lets say so many years later you decide to host the domain for the first time then it will be treated as a new domain.... Its an important but subtle difference to note!!!
That aside domain age as mentioned it gives an indicator... You should also look at the total number of indexed content and other authority metrics... The general correlation being the older the domain the more content and hence the greater its authority metrics such as MOZ DA (domain authority not to be confused with domain age lol), Majestic CF/TF (citation flow, trust flow) which are some good metrics to use with assessing the SEO value of a domain...
BTW - dont bother with PR (page rank) as that has not been updated for sometime and besides it only gave a partial indicator and not defacto that many people still make the mistake of doing....
One great way to check the history of a domain is to use the "wayback machine" - from that you can see the type of content, history etc....
Hope that helps!
Cheers,
Sid
