With the GKIDS release, the question is now academic.
Taeko is supposed to be 27 years old, and her design is supposed to emphasize her status as Christmas Cake. Instead, she inexplicably looks forty-to-seventy years old in some shots, which is honestly bizarre. Is there a seperate underlying meaning to this, or is it simply an awkward visual happenstance?
The "making of" feature that accompanies the DVD release reveals that the design team paid special attention to the characters cheek muscles (something never really seen in anime prior to this film's release) in order to make Taeko and Toshio look to be believably in their late 20's. The above comment highlights how this was perhaps not entirely successful — something the lead animator worries will happen, in fact.
I think they look very believably in their twenties. The problem is, we're so unused to seeing anime characters who actually look like adults and not just like big children. Twenty-seven-year-old Taeko gets noticeable laugh lines when she smiles, and, suddenly, she's forty to seventy? No. We're just too used to the presence of any wrinkles at all being used as a sign of extreme age. In reality, yeah, your face will probably have a few lines, too, by the time you're in your late twenties.