Perhaps there are advertisements that work better for some age groups than others? So by targeting, they can deliver the right ad to the right user.
What we have to reconcile is how to balance the targeting capability and advantage of using that targeting capability against the legal and ethical considerations.
So for example, there's probably a difference between "let's target all of these age brackets with creative that is most relevant to them" vs. "let's target 18-34 men and explicitly exclude everyone else." The difficult part is there's a lot of gray area there. What if one segment performs horribly? Should you exclude them? What if someone has a bias? Can they just run intentionally bad creative or poorly targeted ads against the demographics they don't want to create a self-fulfilling prophecy?
Things are not black and white, but it is encouraging to see the discussion being had.
The thing is that they weren't targeting the "right ad to the right user". It isn't like they were customizing ads to different groups hoping to increase their diversity. They were only targeting one group of people and excluding others upfront. No trying to show the ad to older folks nor investing in an ad for those folks. And even if you produce multiple ads targeting multiple ages, it is probably prudent to simply show them to everyone over 18 in the area you are advertising in - just to be on the safe side of the employment laws.