"You're white and your boyfriend is black, that's gross."
Saying that isn't okay, because it's racist, insulting and downright stupid. Homophobia, transphobia, all of those things are disgusting too. Gender, race, sexual orientation... this stuff has nothing to do with love. Insulting or seeing a couple differently because of any of those things is wrong.
Yet it's perfectly acceptable say "eww, he's old enough to be your dad!"
You probably didn't even think anything when you read that. But it's actually just as insulting as saying "eww, your boyfriend is black".
There's an argument that ageism is okay because you grow out of it. You don't, though. You grow up and still brand that middle-aged guy's 19 year-old girlfriend a golddigger. You still judge the guy himself, joke that he's a pedo, or that he's taking advantage of the girl. A lot of people still don't consider standing up for elderly people on the bus. Even if some people do grow out of it, how does that make it okay? If you grew out of racism it'd still be ridiculous and unacceptable.
Just because you think an age gap in a relationship is gross or wrong doesn't mean that it is. Maybe your friend's niece was immature, her older boyfriend took advantage of her. Maybe a lot of young people making those decisions were naive, they didn't know what they wanted. It doesn't mean that's true for everyone. It certainly doesn't mean that everyone else should have to suffer ageism just because there's some dickheads out there.
There's a lot of young girls marrying older guys for money, but there's also a lot of older girls marrying guys for money. It's equally common, just not seen in the same way. As with all stereotypes, most of the ageism ones aren't even true when you shrug off your prejudices and look at it realistically.
Of course ageism isn't just about relationships. It's about anyone who's affected by it, how old people are thrown into care homes and ignored, how young people are treated with less respect just because of their age. It works in reverse in relationships, when someone really does cast off their partner because they're old or young.
None of this is acceptable. It's as bad as any other prejudice.
It's human nature to judge people, we all do it. But you don't walk up to someone and tell them how ugly they are (unless you're a complete twat). So nor do you judge someone for anything age-related - at least not out loud.
A lot of you probably know that I'm 18 and engaged to a guy who, yes, is more than old enough to be my dad. I've been a money-grabber, whore, husband-seducer, naive kid, you name it. He's had it too; pedo, kiddie-fiddler, too old for that, throwing away his wife for a younger model... we've heard all of them and none of them are actually true. I couldn't care less about his money, his ex-wife's taking it all anyway. I certainly didn't seduce him and unless there's something he's not telling me, he's not a pedo. We're just two normal people in love. This isn't about us, though, it's about everyone. There are lots of couples like us and we all have to put up with that. We shouldn't have to be insulted like that. We shouldn't have to listen to snarky people telling us our marriages will fail. Nor does anyone deserve to be treated differently because of anything age-related.
We can't stop racism, homophobia, transphobia. We can't stop ageism either. But we can stop treating it as something totally acceptable. Please think next time you say "old enough to be your dad" in a negative way, or you joke that someone's a pedo. Stand up for an elderly lady on the bus. Make someone on GDC aware that they're being ageist if they make one of those comments. You might think those things are nothing but they aren't. Just like you can (and should) help people who have to deal with other prejudices, consider helping people who have to deal with ageism, too.