A 20 YEAR OLD CONVICTION WILL NOT HAVE A MAJOR IMPACT ON YOUR LICENSE, AS MARYLAND DOES NOT HAVE ANY ABSOLUTE BAR TO APPLY, BUT IT MUST BE DISCLOSED.
http://www.mbon.org/main.php?v=norm&p=0&c=chrc/index.html

CONSIDER APPLYING FOR AN EXPUNGEMENT, IF NOT QUALIFIED YOUR STATE ACCORDING TO THE AMERICAN BAR ASSOCIATION IS ONE OF A FEW STATES THAT DOES ISSUE PARDONS ON CASES.
Write to the following address and indicate that you would like to have a pardon application mailed to you. Please remember to include your complete mailing address, including zip code: Pardon Application - Maryland Parole Commission -XXXXX Suite 307
Baltimore, MDNNN-NN-NNNN

http://www.courts.state.md.us/expungement/

The Board reviews applicants’ criminal records to determine if the violation is related to the qualifications of a licensee.

The Board may deny an applicant for any felony or for any misdemeanor convictions that are substantially related to the qualifications, functions or duties of a licensee, anything medical related, SEXUAL ABUSE, CHILD MOLESTATION, ETC.

Crimes that the Board considers to be related to the qualifications of a license can be found at The Board of Vocational Nursing and Psychiatric Technicians’ web site and include, but are not limited to:procuring a license by Fraud, misrepresentation or mistake, a conviction of practicing medicine without a license, conviction of a crime involving fiscal dishonesty, etc.

As an applicant, you should present the Board with as much detailed and truthful information as possible. You must report: (1) traffic ticket(s) that resulted in a fine of over $500.00; (2) conviction(s) that have been expunged (i.e., a conviction that was stricken or deleted from official records); (3) all misdemeanors, felonies and/or convictions that resulted in nolo contender pleas; (4) convictions that occurred prior to the age of 18; and (5) military convictions or dishonorable discharge.

If the Board determines that an applicant has been rehabilitated, he/she will be admitted to practice. To determine whether the applicant has been satisfactorily rehabilitated, the Board will examine the nature and severity of the act(s) or crime(s), evidence of any act(s) committed subsequent to the act(s) or crime(s), the time that has elapsed since commission of the act(s) or crime(s), the extent to which the applicant has complied with any terms of parole, probation, restitution, or any other sanctions lawfully imposed, and evidence of rehabilitation.