The ultimate goal of advertising is to target the people most likely to take an interest in what you are offering. What if it is simply that, statistically speaking, younger people are more likely to be looking for work or willing to change jobs?
I certainly don't have data in front of me, but it seems like a reasonable assumption in my mind. The youngest people just starting their careers are highly likely to be looking for work, naturally. Slightly older people often start looking to change jobs to move into higher paying roles. And by the time someone is moving beyond 40 they have reached the top of their career mobility and are happy to stay put at their current employer, plus an increasing unwillingness to uproot their family to move to a new town for a different job. There are always exceptions, of course.
If, for argument's sake, we assume this is true, I can completely understand why they would only want to target those under 40. Statistically, the ads shown to anyone else are just a plain waste of money.