This article, Facebook Job Ads Raise Concerns About Age Discrimination, raises an interesting question of uncertain legality. To be precise, court decisions are not clear as to whether targeting advertisements for jobs to younger workers violates age discrimination laws.
It seems to me that it could be evidence of discriminatory intent, but that if the employer actually received applications from workers of different ages and considered them fairly, that would be the real issue. So if the ads or notices were limited to places where only younger workers would find them, that would be problematic.
Source: The New York Time, 20 Dec. 2017, by Julia Angwin, Noam Scheiber, Ariana Tobin