This phrase refers to the age buffer zone that you are able to perform sexual acts on a female, without it being weird. A person inside your "sexual jurisdiction" us typically a few years older or younger than yourself. Venturing outside of your sexual jurisdiction can result in being ridiculed by friends, or put in prison.
Paul: "Wait your girlfriend is only 16 years old?"
Mike: "Yeah but I only turned 18 last month, so she is still within my sexual jurisdiction"