> The law California (and other states) passed doesn't define what content has to be blocked for which ages
No, but it's a framework that would allow other laws to do so. Because...
> it's not as if they had no idea the children endlessly posting selfies and posting "six seven" on their service weren't adults.
...you can make statements like that which sound like common sense, but it would be incredibly hard to regulate based on "if you know, you know" (or "you should have known"/"you had to have known"). The law has to provide (guarantee) a way for them to know in order to actually require them to take action based on it.
> As a parent, I might think that my 16 year old should be allowed to look up information on STDs but the websites that collect my child's age could decide they can't
This is a different problem. It sounds like you're essentially wanting to guarantee access to certain things, not just for your own 16-year-old, but for everyone else's, too (because if it was just yours, you could look it up for/with them if necessary). It'd be difficult to compel businesses to provide services to audiences they don't want to. But again, that's a separate problem that doesn't necessarily conflict with the rest of the system.