Ageism is the practice of discriminating against someone because of their age.
For instance, if an old man tells you to 'get the F off [his] lawn', then you move the large plastic letter F off his lawn. Simple. But you can't discriminate against him, because he's older and clearly has more experience than you do.
Let's say that a bunch of Social Justice Warriors have shown up outside your house with pitchforks and torches, threatening to burn down your house because you assumed the gender of the nice man (we think) working at the post office.
Sadly, the old man can't help you here. He's just had his coffee.
That, in fact, is not ageism, it's sexism. Which is a completely different box of free-range spiders.
Let's also say that you meet a black person, and then realise he's stolen a car, and threaten to report him to the police. Then he gets angry at you, saying you're only assuming that all black people are criminals. This being despite the fact that he has his stolen car parked outside with no registration.
That's racism, which is also unacceptable, but we can't deal with it here.
Like I said, this is the Ageism Clinic.