Absolute privacy? Not at all, the fact that I'm over 18 is personal information, you've all invaded my privacy a little bit by reading that. Absolute accuracy? Not at all, I have no idea how anyone would ever prove for sure someone's age. Any potential solution is going to about compromise. The real question is: How well can we verify someone's age well enough while preserving as much privacy as possible?

The best solution I've heard of, that hits a pretty good compromise, is giving the local device some indicator of the user's age, and allow applications or websites to perform a limited resolution query of that value, along the lines of which of several age brackets does the user fall into. The birthday can optionally be provided when the device is configured; a parent can set up a device for their kid, setting whatever value they want for the kid's age. A good implementation would make it quite difficult to extract or change that birthday value without admin rights, which the parents would keep.

If this sounds a lot like the laws in the news from California and Colorado, that's because it is. I think that they're stupid laws, but they describe reasonably good features for software. That law making effort should have been put towards banning the incredibly invasive and somehow also incredibly inaccurate use of AI image processing for age estimation.