I don't think so. I know plenty of 40-somethings and none of them could be confused with grandmothers either (other than possibly their kids got married in college and now have babies or toddlers. Yes, technically those are then grandmothers, but AVN or not the look of a grandmother to me is more like latter half of 50s or 60s+ not someone around 40. Hell, I even know of some GILFs in their 60s who look damn fine for their age and who get confused for MILFs.
I think you missed what I meant. In a lot of VNs, the mother looks more like the older sister, or even younger than the real sister. Grandmothers often look more like mothers with gray hair due to the absence of wrinkles and their fair skin.
That's the reason in VNs that portray them 'older' there are comments about them actually looking their age/role.
So if ones expectation is mothers/grandmothers looking one generation younger than they should, it would make total sense to mistake her for a grandmother.
Therefore to me, the problem seems to be the discrepancy in looks between real mothers / grandmothers and porn VN mothers / grandmothers.
Ultimately I'm completely with you. But this discrepancy could explain the different perceptions of that image - depending on ones expectation of a grandmother or porn/VN-grandmother (who often is just identifiable by the gray hair).