Last year(?) YouTube changed TOS to state (IIRC) that if you didn't explicitly mark your own videos as 'not for children' they would be included in search results that included content for young children. Then, if the video was found to violate TOS in the content, it could be removed or demonetized.

Creators like myself who may have cursing or other 'adult themes' would need to check a box saying that the content was not intended for children or risk TOS violation. Ergo many people self-imposed age restriction on their content.

So while people are looking at this in the lens of 'YouTube is censoring free thought!' it is entirely possible (maybe even likely) that the creator of the video chose to exclude children from their audience, and that YouTube's request for age verification is the consequence of that creator's choices.

Obviously there are still troubling implications here, but if a creator chooses to age-restrict their own content that's not really YouTube's fault (though their poor review policies combined with a 'shoot first' approach to takedowns could definitely be seen as strong-arming creators into this paradigm).