1. It is the responsibility of the advertiser to ensure that the advertisement is legal. Facebook is certainly not the employer, nor is it registered or defined as an employment agency, so by the language of the law [1] it does not meet the definition of an applicable party.
2. If the advertisement itself did not _indicate_ any preference based on age, then it may not have violated the law even if it was disseminated according to age.
The second argument is tenuous because (my understanding is that) users could click the ad to plainly see the targeting criteria, but displaying this is not the doing of the employer, which is the only entity to which the language of the law seems to apply.
[1] https://www.eeoc.gov/laws/statutes/adea.cfm "It shall be unlawful for an employer, labor organization, or employment agency to print or publish, or cause to be printed or published, any notice or advertisement relating to employment by such an employer or membership in or any classification or referral for employment by such a labor organization, or relating to any classification or referral for employment by such an employment agency, indicating any preference, limitation, specification, or discrimination, based on age."