Politically Correct Definition: Ageism is defined as being the discrimination against a person or group of people based solely on age.
Real Definition: A moronic belief system held by mentally-retarded fuckshits who actually believe that age is an accomplishment and that anyone who is young is inferior to "adults".
Fond of using the excuse: "you're under 25, your brain isn't developed yet" to win virtually all arguments against otherwise perfectly rational people who just so happen to be in their late teens and early twenties.
These type of people also often believe that their generation is superior to the current one, and that young people are to blame for all of the world's problems. Furthermore, these people think that they are always correct in all situations, due to the fact that they have "so much more life experience" and a "fully developed pre-frontalcortex" according to various bullshit pseudo-scientific articles.
Equally as annoying is ageism against elderly people, which mainly occurs in the workplace. In many countries people over a certain age are expected to be "too frail and senile" to work, even when the applicants are just as qualified as everyone else.
40 year old airhead: "Hey punkass, you on your way to get high and have anal sex with 7 virgins?"
Another PC term for sentiments and practices that don't fit someone's abstract model of perfection. In this case, refers negatively to any distinction made on the basis of age. See racism, sexism, lookism, etc.
Daddy, I wanna drive!
But you're only three! Ageist bastard!
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The senile old fool ran over a dog and a hydrant. Shouldn't they take away his license?
That's pure ageism, you bigoted fuckhead!