Q. When does the date of an article make it too old to be used for my research?

The importance of the date of an article will depend on the research topic. If you are writing about teaching undergraduate students, Chickering and Gamson are two authors who are well known for their seven principles of good practice, so you can use this article. It is a bit old but it is a classic as the principles still apply to students today

Chickering, A. W., & Gamson, Z. F. (1987). Seven principles for good practice in undergraduate education. AAHE Bulletin, 39(7), 3–7. Retrieved from http://files.eric.ed.gov/fulltext/ED282491.pdf     

If you are writing about the retention of staff in the occupational therapy profession, then this article that mentions “a current trend in career literature” and was written in 1993 will be too old for you to use because things have changed a lot in the occupational therapy profession since 1993

Bush, J. V., Powell, N. J., & Herzberg, G. (1993). Career self-efficacy in occupational therapy practice. American Journal of Occupational Therapy, 47(10), 927-933. Retrieved from https://pdfs.semanticscholar.org/20e3/2cb0e1758cc5a029d3d867179859acb33c95.pdf