Why is the 18-49 Demographic so important?

Television programming is tailored to members of its key demographics. Despite the increase in time-shifting to watch recorded television and shows on the Internet, the use of television as an advertising vehicle is still determined by demographic characteristics or who is watching at what time. The subset of ratings that only includes the key demographic of 18 to 49-year-olds is often referred to as the "key demo". The "key demo" group tends to spend more money with brands that advertise on television shows as compared to the younger (very little expendable income) and older (more brand loyal and less willing to change) demographics.