With US-based carriers, FAA regs govern the jumpseat and any other access to the cockpit. The (overly simplified) answer is that you've got to be an airline pilot (working for an airline on an "approved list") in order to jumpseat, or access the cockpit inflight, on any US carriers. Most pilots are happy to welcome pre- and post-flight visitors, though, after essential cockpit duties are completed.
Foreign carriers have different policies. Use the "search" feature above ... just enter the word jumpseat and search the Questions, Wannabes, and Aircrew Notices Forums. You will find loads of jumpseat information in the threads that show up.