"The Age Discrimination in Employment Act of 1967 (ADEA) protects certain applicants and employees 40 years of age and older from discrimination on the basis of age in hiring, promotion, discharge, compensation, or terms, conditions or privileges of employment." [0]

OldGeekJobs.com works since it's the reverse of this... essentially discriminating against younger candidates. It's not legal to discriminate against 40+ in the US.

[0]: https://www.dol.gov/general/topic/discrimination/agedisc