The universe dates back billions of years and perhaps is the oldest thing ever to be radioactively dated. The earth is also 6 billion years old and is among the list of old specimens dated. The other living samples found on earth are all dated to ears less than that found for the actual earth.
It can be understood how the earth was dated and found to be so old. The specimens that walked the earth then died and were buried in the earth for thousands of years before geologists uncovered them and found a way to find their age using radioactive dating.