If they didn't want it, they would have lobbied against it.

I don't think that is a problem for Microsoft, especially because they would want your ID and age, tracking you as much as possible already. Also you don't just change the Kernel just like that, there are licenses attached, drivers and other stuff. From technical point of view, in current implementation it is probably impossible. Let alone Microsoft would face issues with compatibility. If we put the technical difficulties aside, I don't even think Microsoft is against this law and pretty much wants it.

What expense? Adding one more thing to track about you, especially something they already know and essentially just need to verify, cannot be that expensive.

To be weighed against the profits from having all that new data on user identities.

Has Apple fully open sourced macOS?

https://support.microsoft.com/en-us/accounts-billing/manage/verify-your-age-microsoft-account