There are federal laws that apply to employers with 15 or more employees and that prohibit discrimination against any protected class of individual. These laws dictate what you can and cannot say in a job advertisement. Some state laws apply to even smaller employers. In addition, many newspapers make it a policy not to accept ads that are discriminatory, regardless of how small your company is.
Gender-referent language. Federal law prohibits you from making statements or implications about not wanting people from protected groups (i.e., members of a certain race, color, ethnicity, national origin, religion, gender, age (over 40), disability, or veteran status). A very narrow exception applies to jobs where there is a bona fide occupational qualification (BFOQ) for the position, meaning that religion, gender, or national origin factors are "reasonably necessary to the normal operation of that particular business or enterprise."
The law does not prohibit you from using titles that seem to specify one gender over another. We recommend that you not use them if you can avoid it. Most can be changed to a gender-neutral form, anyway.
While the law cannot keep you from using gender-referent words and titles, the Federal Equal Employment Opportunity Commission will not permit the use of the phrase "Equal Opportunity Employer" if you do. You must, in that case, choose one or the other — either get rid of the gender-referent terms or do not include the EOE phrase.
Age discrimination. You also have to be careful not to use language that will discriminate against potential applicants for your job because of their age. The EEOC gives policy guidance on how job advertisements may violate the Age Discrimination in Employment Act (ADEA).
Although the prohibition against job advertisements that discriminate on the basis of age is pretty straightforward, the EEOC finds that a case-by-case assessment is needed as to whether the language of a particular advertisement, as well as the context in which it is used, would discourage persons in the protected age group from applying for the job.
For example, the mere presence of "trigger words" (words and phrases that refer to age such as "recent college graduate," "young executive," "athletically inclined," etc.) does not alone constitute a violation of the ADEA. In order to determine whether an advertisement is discriminatory, the ad must be read in its entirety, taking into consideration the results of the ad on the employer's hiring practices.
To illustrate these concepts, we've provided a case study from the EEOC.