From the July 2013 issue of Entrepreneur
Unless your union contract states differently (e.g., promotions must be based on seniority), you can select anyone you like for any job for which you feel he or she is qualified.

However, you do have to be cognizant that people 40 and over are protected underThe Age Discrimination in Employment Act of 1967 (ADEA). If you employ more than 20 workers, you are subject to the ADEA, which protects individuals who are 40 years of age or older from employment discrimination based on age.

The ADEA's protections apply to both employees and job applicants. Under the ADEA, it is unlawful to discriminate against a person because of his or her age with respect to any term, condition or privilege of employment--including, but not limited to, hiring, firing, promotion, layoff, compensation, benefits, job assignments and training.

An exception to the above would be allowed if you cite a legitimate, bona fide reason why only someone younger could do the job. This is not likely to be the case in a job in which many workers in this kind of job are older than 40.