If it is possible for a 20 year old to marry a 17 year old from
louisiana without parental consent or knowledge of marriage.

Dear Berrydj27,

Thank-you for your question. In answer to it, the 17 year old would
require parental or judicial consent but the 20 year old would not.

Under Louisiana State Law if either the groom or bride to be are below
18 years then the person below 18 years would require either Parental
Consent.

"The legal age to marry is 18 for both sexes. Minors must present
written consent of both parents or custodial parent. Common law
marriages are not recognized in Louisiana. "
http://community.nwlouisiana.com/la_info/state_service.htm#mar

ALSO

"(b) If one or both of the parties are minors, the written consent or
authorization of those persons required under the Children's Code to
consent to or authorize the marriage of minors."
Form the Loisiana State law on Marriages
http://www.lna.org/law/marriage.htm


See Also:
http://www.law.cornell.edu/topics/Table_Marriage.htm - This lists the
Age consents for Fifty states (including Louisiana), The District of
Columbia and Peurto Rico

I hope this helps. Please let me know if you require further
clarification.

For the Search on Google I used the Serch terms "Louisiana Marriage
laws Age consent"