The concept you're describing is a subset of Market Segmentation, specifically age-based segmentation.

It became widely accepted after it was formalized by Smith (1956) and is now a staple in any marketing course.

The idea existed before though, albeit not formalized as such. Here's an example of this, describing the German book trade in the 19th century. I wouldn't be surprised if it existed even (much) earlier, though I currently don't have the time to find any more sources.

To answer the question as to why, just look at Smith's definition: "Market segmentation is based upon developments on the demand side of the market and represents a rational and more precise adjustment of product and marketing effort to consumer or user requirements".

In the case of seniors, this implies they are significantly more likely to spend if they receive a discount. The actual reasons behind this don't have to economical, although they are sometimes framed as a means to fight old age poverty. Here's a paper that specifically deals with this issue.