>At eight they have limited comprehension of the world around them and limited language skills.

I have two kids and can confidently say eight year olds generally have good language skills, are capable of expressing themselves just fine, and have good comprehension of the parts of the world that they've been exposed to.

So they can conduct a nuanced debate then?

Mine couldn’t until they were much older. And I have more so perhaps that’s more statistically valid?

Can they identify easy stylistic device, like an extended metaphor or an anaphora?

Because until they do, I will consider their comprehension skills limited.