Cases when old references are valid?
Taken at face-value, these guides are useless or even harmful, as they would promote an organized loss of memory and lead to multiple reinventions of the wheel.
It is also important, however, to show that your research is moving at the cutting edge of the field. This is probably where the advice you mention is coming from. You should demonstrate the topicality of your project by discussing the most recent literature to the extent that it is adequate for your research problem. Strategically, it may be useful to highlight your project's contribution to the latest research, including new trends, especially in the introductory section. But take care not to exaggerate this tactic, as it cannot compensate for substantive shortcomings and might even come across as superficial.
To explicitly answer the question in your title: References remain valid until superseded by later research with respect to the aspect they are cited for. To make that judgement, you have to know the field; to know the field, you have to read. Looking at the age of the reference cannot replace making that judgement, it can only give you a rough indication as to whether the reference is likely to have been superseded. The fact that different (sub-)fields move at different speeds makes it even less reasonable to put a number on a reference's shelf-life.