According to the lecture how old are the sources that we are allowed to use? And what are the various sources we are encouraged to used in order to have a variety?

According to lecture, it has been said that in doing the literature review, only updated resources should be used. And more precisely, it was recommended that compiled resources for literature review should not be older than ten years at the time of research. However, there is an exception that resources can be used older than ten years, when there is absolute necessity like using any theories or historic documents. Moreover, it is recommended that researcher must use variety of resources in the literature review. The researcher should come up with mixed resources like using books, journals, magazines, newspapers, archival documents & records, thesis as well monographs of published dissertations. The blog and opinion articles as well as resources like Wikipedia should be avoided in the literature review.