It could be argued that actively filtering a job advertisement to be shown to younger people only is similar enough to allowing the ad to be seen by all ages, and then filtering out older applicants when they inquire about the ad. You're just moving the "age filter" to a different step of the process.

Another scenario: company creates company-jobs.com, which contains all their job postings. The homepage requires you to select your age range before accessing the listings, and only choosing "18-30" results in them being displayed. You're acquiring the individual's age range in advance - exactly the way an ad network does - in order to decide whether job listings should be displayed to a person belonging to that age group.

Direct analogy: the ad network is operating as a recruiter for the company. Recruiters are not allowed to discriminate based on age. End of story.

This is not the case in USA, under the legal theory of "disparate impact".

https://en.m.wikipedia.org/wiki/Disparate_impact

That does not apply to advertising.

It could apply to say, requiring candidates to provide credit scores which disadvantages a protected class.

> But that in no way applies to advertisement

Are you sure that's true? Quite a few job-boards exist purely so large companies can show they've explicitly attempted to target veterans and so on.