No what is that?

So how would you age them up?

People who aren't familiar with the artstyle misread the ages because they try to apply their intuition, which is based on the way actual people look.

Which doesn't work, because until the character being drawn is someone "old", age-indicating details on the face are omitted.

They look young because the eyes are big and skin-detail level is low, but it's not that they're drawn to look young, it's that a lot of facial detail is just not drawn in anime.

Once you add even a little bit of those age-indicating details you quickly land in the territory of how the elderly are drawn.

There isn't much of an in-between.

It does exist, but you kind of have to see enough anime/manga to start somewhat "accurately" guessing the age of characters based on how they look. Even then it varies between artists, so a character that looks 20 within the context of one series would be a child in another, or vice versa.

The age is 100% in the size of the head relative to the body, and in the size of the eyes, relative to the size of the head. But it's completely relative. The ratios that indicates a given age is completely up to the artist so it varies wildly from series to series.

Amen