Not that I saw anything indicating that the Mito was Kushina's mother either but age based arguments don't necessarily hold since a point was made to state that...
If the village members life spans were extended who's to say their child bearing years weren't also extended making it possible for them to have kids longer than normal. Mito apparently lived from the time of the founding until at least the time the Fourth was a kid.Originally Posted by Chapter 500 Page 03
That said from the way things were described it seems like Kushina knew of Mito as a respected elder but wasn't related to her by anything other than whatever distant bond related the members of their clan.