I was 45 when I finished my M.A. in Anthropology with a specialization in Archaeology. I am not sure what your background in school is but many starting jobs as an archaeologists require taking a field school or at most a B.A. Granted many of those are of the shovelbum variety but that experience can be built into an excellent CV that can assist you in securing a permanent full time job. If an M.A. is really your goal then start getting published in peer reviewed journals, try to volunteer for NSF funded projects, develop relationships with professors you want to work with for your MA. A low GPA is not the end all and be all of an application to grad school.
One more word of advice is to really screen your potential mentors for grad school, There are many great universities that offer terminal M.A. , you have options.