In the magazine case you are taking out an advertisement with no ability to discriminate. In the Facebook case there is a specific setting based on age to show job ads. Job seekers over the age of 40 are protected so this is discrimination against a protected class. The problem is not that young people were targeted, the problem is that those over 40 were not.
The only true way to solve this is to force by law to hire older people (e.g. 30% of your workforce must be over 40); otherwise all this proxies like trying to regulate third-party ads and else its just an endless cat-and-mouse game.
Job ads have no age options allowed.
Fixed.
A US Supreme Court case comes to mind, Batson v Kentucky:
https://en.wikipedia.org/wiki/Batson_v._Kentucky
In which it became illegal to dismiss a juror based on race. A simple, sensible fix, which most agree did very little to stop race based discrimination from juries. Doing so just became a bit more complicated.