I think this is a reasonable balance without being invasive as there's now a defined path to do reasonable parenting without being a sysadmin and operators cannot claim ignorance because the user input a random birthday. The information leaked is also fairly minimal so even assuming ads are using that as signal, it doesn't add too many bits to tracking compared to everything else. I think the California bill needs a bit of work to clarify what exactly this applies to (e.g. exclude servers) but I also think this is a reasonable framework to satisfy this debate.
I've seen the argument that this could lead to actual age verification but I think that's a line that's clearly definable and could be fought separately.