With California's AB1043, this was on my mind, although wasn't specifically about that law. Generally, giving users more control is a good thing, esspecially when it means excluding potentially distressing or harmful content. In general, having filtering settings like this provides a way for users to pick and chose what they want to see. While I don't think an age value is the best way of implementing it, I do think it is likely to be better than having nothing at all.
So long as its only a local value, the only significant downside I see, is its use for fingerprinting and tracking. This is an issue, but being only one number, is relatively inspecific and unreliable. User agent strings provide far more data, and are far harder to manipulate meaningfully, for example. Furthermore, so long as its all managed locally, privacy focused software would also have the ability to either not provide the value, to use brackets in UI rather than a asking for a specific number, or to just use a default value, like 99. Given that, it seems like an age flag would be just another in a sea of fingerprinting methods, while the convenience and utility provided could be significant.
Ultimately, I feel like a series of boolean flags for different subject matters to filter would be better, but because an age value seems closer to being implemented, thats my focus.
So, having a local, "age" flag used for filtering content isn't a bad idea.
Change my view.