What Udmas posted is not entirely accurate. States and counties have their own variations of the law... here are Maryland's, including some in the surrounding area. With that said, the OP was not incorrect in posting age in her advertisement. Hope this doesn't keep you up at nights BMIC.
While the classes of persons protected by the advertising discrimination laws are also discussed above, it is worth summarizing them once again here:
Federal Law: race, color, religion, sex, handicap, familial status, national origin
D.C. Law: race, color, religion, national origin, sex, age, marital status, personal appearance, sexual orientation, family responsibilities, physical handicap, matriculation, political affiliation, source of income, place of residence or business.
Maryland Law: race, color, religion, sex, familial status, national origin, marital status, physical or mental handicapVirginia Law: race, color, religion, national origin, sex, age, marital status, disability.