Like most things in life, it depends. The age of a source often matters in relation to the topic at hand. For example, if your topic is historical you can generally use older sources. If your topic is something more contemporary and changing by the minute, you should use newer sources.
Solar energy would seem to fit in the latter category, but that does not necessarily limit you from using a source that is five or more years old. I would ask, is five year old information about solar energy good enough? If you were using the source to demonstrate the basics of solar energy, your probably safe. If on the other hand, you wanted to illustrate the latest and greatest developments in solar energy, a five year old source might be too old.
Matt Bejune
Coordinator of Library Reference and Instruction