Consider a Source's Publication Date
It's very important that the information and evidence you are planning on presenting come from sources that are as reliable, up-to-date and relevant as possible.
Information regarding the most recent discoveries, technical advances and developments in all fields of endeavor are published on a regular basis and you should include them in your research.
If you cite five-year-old sources regarding procedures for treating AIDS, for instance, your research would not include the latest findings and treatments in that field. Your information would be dated and not necessarily valid.
When using older materials, do so when their value has held up over time or your research paper specifically calls for the inclusion of an historical perspective.