As in our society, becoming old is portrayed as the worst part of life. There's ads of makeup to reduce fine lines, plastic surgery to appear more youthfull and a saying, "Ladies never reveal their age." Aging is apart of life, it's inevitable. But it's something people don't like to talk about, yet reveal it. So, I think the stigma about old people comes from the fear of aging. So, people tend to think of the elderly as gross and useless. Which is absurd to degrate a class of people just because of age. Although, I most of this mentality comes from mostly teenagers. Problobly arising from the age differences the two generations have between them.
Hope this helped.