In all industrialized countries under capitalism, when elders retire, and stop being useful to society as producers, they are seen only as consumers. In addition, since they tend to consume less, they are often cast aside, more or less forgotten.
In USA this situation is aggravated by the special circumstances under which this country was founded. When they decided to cross the Atlantic, most of the early settlers had to leave their aged relatives behind in the old country. In some cases they were no doubt happy to be free of domination by elders, who tended to run things, both in church and state as well as in the immediate family. Once here, as settlers moved on west in stages, they would often, again, leave their elders behind. As a result, the three-generation residential family was never the norm among white Americans, although it has always been so among the Native Americans, as well as in most other countries. Even in those countries where the elders do not actually live in the same house with their offspring, they often live close by, where they continue to exert a strong influence on the larger family. They receive a certain amount of respect and attention from their own descendants and from society in general.
Another factor peculiar to USA has been the premium that needed to be put on sheer physical energy to subdue the land, thus devaluing the contributions that older and weaker people could make to society. In fact, the urge to start anew, with fresh ideas, in a new land, precluded the consultation of elders in the search for solutions to problems never met before. Without precedents to follow, tradition does not play a large role.
Gradually, as USA society under capitalism began to develop a social conscience, the disgraceful "poor farms", to which indigent elders had been relegated, were replaced by retirement centers, convalescent homes, geriatric wards in mental hospitals, and even elegant gated cities and villages for senior citizens in what I call "golden segregation". Although provision for elders is now more adequate, they are still effectively left out of the main stream of daily life.
Once they have been thus so efficiently isolated, I think that this segregation is the main cause of the ensuing oppression. When we do not see a given group of people on a daily basis, we tend to get strange ideas about them (from lack of information). These are passed on unthinkingly and eventually become stereotypes. Significantly, small children who have regular contact with elders do not see them as "old", even though they may see other older people that way.
Since most all of us in this country are caught up in the youth cult, we tend to think of youth as the norm, and being young as the highest good. So, when a younger person does manage to meet an elder with whom she finds a common bond, and tries to bridge the age gap, all she can think of saying is, "Oh, you look (or act) so young!"--in other words, "You are really one of us--you don't belong with those old fogies."
A psychological cause of oppression can be related to adultism, of which it may be a by-product. Those of us who were abused, neglected or overpowered by elders as children may then feel antagonistic toward all gray-haired people.
Whatever the reasons, oppression of elders can have devastating effects. If the only way to be recognized and validated is by appearing young, some go to great lengths and expense to create the illusion of youthfulness, or to keep a job. Think of all the money spent on creams and operations to eliminate wrinkles, hair-dyeing, wigs, face-lifts, breast operations. Most of these means involve women, but men also suffer by trying to keep up physically with youth. And what about Viagra? After all this, it is not surprising that the oppression has become so internalized that elders themselves tend to validate each other in terms of their youthfulness.
I have noticed a special aspect of ageism that is combined with sexism, and could be thought to have a biological basis: older men are usually attracted sexually to younger women, presumably because they can reproduce. The reverse has not been true, because older men can reproduce. For millenia this behavior was vital for the preservation of the species. However, such is no longer the case. In fact we now have the opposite problem: overpopulation. But, older men continue to prefer younger women. I think this is an attitude left over from the past, not biologically innate, and reversible. One factor that is already counteracting this trend is aging movie actresses, instead of fading away, or making themselves up to act in younger roles, are finding roles for their real age and continuing to be attractive role-models for women.