Is it illegal to lie about your age at the time of marriage?

Yes, you are falsifying legal documents. In some states there are specific penalties for getting married under false pretenses and even the individual performing the wedding can get into trouble. *Only if you are underage, in which case the identification documents required would be false and that would be the fraudulent act. FYI, all state's require proof of age, preferably a birth certificate and other documentation such as Social Security card, driver license or state ID, passport, etc. before a license will be issued.