This is exactly the sort of infrastructure that would make it super easy to pass a law banning tracking and advertising to minors. Once every platform can trivially detect when they should turn off the ads there's no reasonable counter-argument about privacy or feasibility.
Have we banned advertising tk minors on tv shows aimed at minirs? Are barbie/action hero/etc commercial showing kids having fun with barbies on channels whose primary demo is children no longer a thing?
Technology has never been tge limiting factor. Politicak will is.