Missing birth year of a domain

Maybe the domain was adopted and there's no record and therefore not showing the birth date. :D

Depends on web crawlers, also domain may have just sat rather than been parked, so that when you visit it's just a blank page. It could be it was going to be developed then wasn't. Or it was not forwarded to any other domain. Also may have been dropped then reregistered. That is where the dates look all messed up.
I wouldn't worry about that, the main concern would be was it used for adult content, was it an old spam site did it have a reputation for spyware/virus etc. You can search out a page history reputation etc. If you find nothing bad then