I strongly disagree. There are no kids in my household, and no one else ever uses my devices, so the idea that I need to prove that I'm not a kid for me to be able to pay bills or file taxes on a stock device is ridiculous.
Is anyone proposing age verification to file taxes? I'd hope you already have to provide some sort of stronger proof of identity to file a tax return anyway.
I don't understand the force of that argument. Where is the "so" coming from?
"Age restriction" can be implemented where stock devices are completely unrestricted. For example, just with better parental controls and education: children don't have access to property, so the parent takes their device and enables the parental controls, but you own your device so leave them disabled.
And this wouldn't affect Linux or FOSS: on a child's device their parent installs either a proprietary OS or a FOSS with parental controls, but again, on your device you install whatever you want.