About how old was she when she swallowed Yatagarasu's corpse? Can something like age even be applied to youkai? do they ever grow older?

That's their size, not their ages... Check the chart on the background

so where's 3 and 4?

Gah! said:
That's their size, not their ages... Check the chart on the background

It's more like a proportion chart, i.e. proportion of head size to the rest of the body.

I'm disappointed about how serious every size looks compared to 6.