The problem is that you could face liability if you do business in the United States and permit a minor in California to use an OS in non-California-compliant mode. If you're an "OS provider" in Wichita, KS, California will find that its jurisdiction still applies because the minor was in California and sue you in its courts. If you fail to turn up that's a judgement for the state by default. (And if you do turn up, it's a judgement for the state as soon as they prove a kid ran your non-age-checking OS.) And, thanks to the "full faith and credit" clause of the Constitution, California will be able to collect on its judgement against you in Wichita.
Hardware vendors are not going to want that kind of liability, in California, Colorado, New York, or anywhere else. So they will switch to selling hardware with locked bootloaders and only allowing approved operating systems within that locality (which for end-user PCs will mean pretty much just Windows). There is still foreign hardware, but those chinesium PCs are going to be confiscated by ICE unless the Chinese manufacturers also play ball.
Besides all this... federal legislation is coming.
If you'd humor me, or just read the last paragraph for a tldr...
So let's say a PC builder(an individual; not a company) were to donate a PC to charity. Let's say it's built with a fairly recent MSI motherboard(https://www.amazon.com/dp/B0BRQSWSFQ/) 'MSI PRO B760-P' if you'd prefer to avoid amazon.
I remove all my internal SSDs and NVME drives but buy a new 1tb SSD for whoever receives the PC. I also install a Linux OS, as well as sign the secure boot keys via sbctl myself, setup ukify, efibootmgr, etc. Everything the recipient would need to switch over to another OS if they so choose.
But oh no, the donated PC landed in the hands of Johnny, a 17-year old in California.
So who's at fault here, MSI for creating a BIOS that allows for non-windows EFI images to be installed? The PC Builder(donator) for knowingly installing Linux(though not knowing where it would end up)?
This is kind of what confuses me and I'm curious what this means for future hardware sold in the US and those who build PCs for their own use or others. Most modern motherboards are "locked down" by default, but can easily be unlocked by the end-user, it may take a few extra steps or be a bit harder to find but still pretty simple for someone moderately tech-savvy.
The full faith and credit clause does not apply if the court lacks jurisdiction, which California clearly would. There's a reason "California compliant" already exists as a phrase; you can buy and sell things that break California law outside of California. If you bring it in that's on you.