Young people are more likely to be looking for jobs than older people, and thus click job ads more. [1] Higher click through rate means much lower cost per click. Based on the numbers in my stat link, you could likely net the same amount of new job applications spending $3000 on millennials as spending $9000 on other demos.
This is a user friendly feature - younger people want to see these type of ads more by far. By the data, older people do not want to see these type of ads.
Nothing to do with worker effectiveness or how HR treats older applicants.
[1] " About 60% of Millennials are currently open to a new job opportunity and are by far the most likely generation to switch jobs. To support that, 21% of Millennials in 2016 reported switching jobs within the past year, compared to roughly 7% of gen Xers and other non-Millennials. " https://www.forbes.com/sites/larryalton/2018/01/22/millennia....