This post assumes that a meaningful amount of defed instances are caused by simple lack of agreement. Often, it’s an orthogonal matter - it boils down to instance A actually understanding something about the userbase of instance B and saying “I’m not dealing with this shit, it’ll make the instance worse for its own users”. For example: the typical user of B might be disingenuous, or preach immoral prescriptions, behave like a chimp, or be a bloody stupid piece of trash that should’ve stayed in Reddit to avoid smearing its stupidity everywhere here.
Are instance admins too eager to pull the trigger for defed? Perhaps, in some cases; specially because it handles groups of users instead of individuals. But those cases are better addressed through actual examples, not through a meme talking on generic grounds.
Brain plasticity, window of opportunity, it’s all babble. You can learn new languages just fine as you age; the matter here is how much time you spend using the language.
The reason why adults perform generally worse than kids learning languages is mostly motivational, and not spending enough time with the language. But as an adult you got access to a bunch of resources that kids wouldn’t, such as a decent grasp of grammar on theoretical grounds, that you can (and should) use to your advantage.
Note however that watching sitcoms will likely not be enough to get any decent grasp of any language. (Otherwise I’d be speaking Japanese, given the amount of anime that I watch.) You’ll need proficiency on four levels: hearing, speaking, reading, writing.