A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
As I replied to Ivy Guo, there is no option to add a title to a Legend. What you describe wanting to do might best be handled by use of the Chart Title to indicate what the chart represents, such as "Comparison by Age Range".
Another option would be to add a Text Box to the chart to serve the purpose, but in my experience excessive labeling contradicts the fundamental purpose of a chart... concise visual representation of the data.