based on age
. but let's
look at other forms
racism : generally seen as a completely UNACCEPTABLE form of discrimination in today's society. There are few people who openly spout racist epithets. Those that do are often called "Nazis" or here in Germany "Faschos". Because of the events of history, racism has been largely phased out. (Don't worry - I know there exists countries with racism remaining in full force and that even in the US and other so called "developed" countries, there are remnants. But for the most part, thinking, open people have decided racism is "out".
sexism : the next step. Just as the civil right's movement in the United States was intended to take down racism, feminism attacked sexism. With sexism, perhaps, open jokes and derogatory comments are more acceptable but they too are on the way out.
Then for example, there is discrimination against gays and lesbians which should be the next "front". The topic is still so heated and formative that it has not been given an "ism". Homophobia, I believe, is a misnomer. As a member of this X-ist class once remarked in my presence, "I don't hate 'em; I just can't stand 'em."
Where does all of this leave us? In general, there is a trend which I personally feel to be ethically-justified and which tends towards a taboo of hate and discrimination. But, ever overlooked is the concept of age. (Here I will address that discrimination against younger people since that is what I have experienced and know. The other end naturally exists and is a direct if partial cause for a loss in societal structure.)
The general case is that since, for example, high schoolers are younger their opinions are unimportant. Basically, generalities are strewn which allow for the complete exclusion of an entire "class" of people; this is exactly the method used in the past to keep slaves, women, and other repressed persons "down".
Whether such ageism will ever end is doubtful, simply for the reason that those who come to power are older and therefore have the "right" to judge who may speak. The effects of said ageism are that the innovation of a society is squashed, thereby reducing new information. (Just as ignoring the elderly leads to a loss of existing information.) And as far as I'm concerned, ageism is just a dangerous and close-minded, and just in need of societal removal.
For an example please see Dman's reply to TallicaBurton in right to bear arms.