Agreed. It is adiaphora - not necessary for salvation.
They weren't there 6,000 years ago either.............
From your link:
So in a general sense, adiaphora means those Christian teachings which are neutral, things that are neither commanded nor forbidden in Scripture. An example of this might be what color of carpet to have in a church, or what time of the day a service should be held. Sometimes there is debate within Christianity on what may or may not be adiaphora.
I don't think this issue is an adiaphoron. While the 6k age is something man-made in that it isn't commanded or forbidden in scripture, creation is described there.