> There are normal developmental milestones in childhood[1], it's a term of art. For 5year olds it includes things like "Speaks in sentences of 5 or more words, and with all parts of speech", or "shows less aggression".

Yes, milestones for western cultures.

> This has very little bearing on things like brain development, or language acquisition. Childhood play is evolved behavior.

Cultural context has little bearing on brain development or language acquisition? Sorry, that's crazy talk. I was raised in a bilingual household, so by any quantitative metric I had twice the language acquisition of all of my peers when entering school, and my nieces and nephews are actually trilingual. Culture has a huge influence on milestones.

"Brain development" is too vague a term to meaningful respond to. Also, "childhood play" doesn't mean "play only among children" it means "play with children". Adults can easily play with children. I do it all of the time.

> I could keep going but I almost can't believe someone here is arguing that peer interaction isn't vitally important for childhood development. Do you actually believe that?

Yes, I have seen literally no reason to conclude that children must interact with other children to grow up healthy. As for your citations:

> [1]https://medlineplus.gov/ency/article/002456.htm

A review of normal biological maturation of homo sapiens (uncontroversial), sprinkled with a plethora of cultural expectations around maturity needed for schooling and other western expectations which neither hold for other cultures, nor are they some kind of mark of a pinnacle of human growth. I could easily see some of these milestones change in the next 20 years as we better grasp what's happening during human maturation.

> [2]https://www.tandfonline.com/doi/abs/10.1080/13670050.2020.17...

Its own synopsis says this is a qualitative study. That's little better than an anecdote to pique curiosity for further study. It proves nothing substantial, and certainly not that this child wouldn't have learned even faster if interacting only with adults, who naturally have an even better grasp of language than children.

> [3] https://www.sciencedirect.com/science/article/pii/S004727272...

This does not prove your thesis, quite the contrary. Per the abstract, "High school students from low-income neighborhoods suffer a learning loss of 0.4 standard deviations after a one-year school closure, whereas children from high-income neighborhoods initially remain unscathed."

In other words, children that have parents with good jobs (or a parent at home), or other adults that are around to supervise and interact with do just fine, it's only children that are left to their own devices that suffer learning loss. Where is the evidence that peer interaction is critical, which is what you claimed?

There is no evidence that the "great equalization" effect of school has to do with peer interaction at all, it's equally compatible with the hypothesis that poorer kids do better in a more supervised environment where they have to act within certain boundaries enforced by adults, which they otherwise wouldn't have with parents that are abusive, neglectful or have to work too much to put food on the table.

There's also considerable evidence that after school programs reduce crime because they spend less unsupervised time with peers, which reduces delinquent behaviour. In other words, the more time children spend with responsible adults the better off they tend to be, and the more unsupervised time they spend with peers, the more delinquent they tend to be.

So in conclusion, I see no strong, causal connection between peer interaction and healthy development. Positive human interactions matter of course (preferably diverse), but not necessarily peer interactions, which is what you are claiming.