If you can target software engineer on Facebook, then clearly you'd just do that.
If you're targeting more broadly for some stupid reason, then there's an economic incentive to target unfairly by demographic, just to narrow down the sheer number of irrelevant ad views by any means possible.
I doubt that you could do that on Facebook. But on LinkedIn, you could obviously target people who claim experience in software engineering.
As you say, that might well effectively select for younger men. And as far as I know, that wouldn't be illegal. But you can't target by age, gender or disability. Because it's explicitly illegal.
