It instantly reminded me about
this recent discussion. Mio is only 35 here but looks much more mature. It's not limited to JAV though (e.g.
Arabelle is only 35 as well but has been
listed as a MILF multiple times already... and in the old days, 40+ women were considered "old women" already simply because people lived shorter lives on average; also, see Vsauce
video on "old" photos)
I think it's more of a combination of individual characteristics, style in clothes, makeup and accessories, as well as some basic patterns like "bigger breasts = more mature" or, conversely, "acting like an airhead = young" etc.), and the overall emergence of the MILF genre is what we all should be thankful in porn: mature actresses can perform much longer, and we get to see at very different ages
So, a combination of historical, subjective and other phenomena is what makes up a person's age that might be very different from their biological age
Also, maybe it's just a marketing move: to create controversy or simply to show her in this new role