Is a 6x4x5 mm lymph node on back of neck at base of head that has been there over 20 years anything to worry sick over?

Not after 20 years: if you have had a lymph node for over 20 years and it has not changed you have nothing to worry about from that lymph node in its present situation. A lymph node goes up-and-down with infection but is sore when that occurs. A lymph node that changes and has no pain associated with it should be checked. Be sure it is a lymph node and not a cyst which is also benign.