This phrase refers to the age buffer zone that you are able to perform sexual acts upon a female, without it being weird/illegal. A person inside your "sexual jurisdiction" is 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.
Dave- "Wait dude, your girlfriend is only 16 years old???"
Nick- "Yeah but I only turned 18 last month, so she is still within my sexual jurisdiction"