It's true that generalisations and stereotypes are used about every age group, but that's different from ageism - and it's older women who get it in the neck, every time, in almost every sphere online.
I don't just mean the general "old trout", "old biddy" stuff in mainstream media.
In feminism, the second-wave feminists whose won many of the legal rights women enjoy today, and who put in the hard graft to get refuges, rape crisis centres, etc set up, are derided as prudes, dried-up sex-negatives, "whorephobes", irrelevant, and so on. I have seen younger women online (especially the ones who try really hard to be More Edgy Than Thou) say with an utterly straight face that, never mind, these old women will die soon so we won't have to worry about their outdated politics that much longer.
And in advertising - if I ever hear "not your grandmother's sewing circle" used as a strapline for some event just once more, it will be one time too many. I see it all the time as a shorthand for "this is so edgy it will blow your mind", and my inward response is that the people writing it would have conniptions if they knew what their own grandmothers and great-grandmothers had got up to - and the risks they took, and the scandals they braved, and the amazing changes they brought about - decades before these twenty-somethings were born.