Just spitballing, I'd tailor my messaging to women aged 30-45 to address our position on helping people return to the workforce, while for over-45s I'd talk about our open policy towards hiring older workers. For each, I'd probably choose photography that featured a representative individual. Yeah, I'm totally stereotyping the audience, but I'm also addressing common issues for jobseekers in those age groups.

To be clear, I don't think that's what these companies are doing or they would have spoken up in their defence. And you also have to be careful that you express openness to, rather than preference for, a certain type of worker.

That's pretty much what Facebook said in it's rebuttal, linked in another comment.

Cost per click is going to be vastly lower, since young people look for jobs much more often. You'll get applicants at a third of the price.

Campaign budgets are limited and you need to optimize your ads so they show up for demographics most likely to respond and be accepted.