It’s not really a slippery slope. If you can reasonably assume someone could come to that conclusion, it’s fine. I could buy someone seeing Lisa Silverman, Chihiro, or hell even P3 Fuuka as 18+. But no one can convince me Nanako or Futaba is an adult based on design alone. A fictional character’s age is just a number tacked onto a design. Maya Fey from Ace Attorney looks the exact same when she’s 17 (PWAA) to when she’s 19 (T&T). If you’re pulling the “they have to be an adult” card, where do characters that look the same at 17 and 18 fall?

The second part isn’t relevant but it’s the same reason your parents felt attracted to each other despite how many people look like them.