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-frontal cortex" 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?"
18 year old: "No... I'm going to a job interview"
40 year old airhead: "Job interview!? At 18? Little children can't have jobs! Your developing brain isn't mature enough to handle the responsibility!"
18 year old: "Dude, that's ageism. I can work just fine. Where in the world did you hear that my brain is "still developing?""
40 year old airhead: "I read it on a news article! Cause I'm an informed adult!"
18 year old: "Who did the research for the article?"
40 year old airhead: "MADD! They've been a reliable organization for decades! What they say must be true!"
18 year old: "Ah, MADD huh? So that's why you're such an uninformed prick."
What overly-formal scientists call humans.
"Anthony! Did you know that Homo-Sapians originated in Africa?"
"Dude, just say humans."