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!
* * *
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!