My favourite twitter comment on this was: "I guess intersectionality doesn't apply to mothers."
I'm not anti intersectionality. One of my favourite writers is Anne Marie Smith, who was writing before intersectionality emerged but definitely engaged in developing foundational thinking. Importantly, her writing was developed as a critique of the way existing discourses, and their notions of the political subject, acted to silence certain politics/people.
And that, I think, is where intersectionality is powerful - when it is a critique or a tool against silencing. Where it is incredibly destructive, is where it becomes a means of silencing itself - usually because it's been caught up in the destructive dreaming of trying to find the most 'politically pure' position from which a political subject/person can speak, imposing a fantasy of a hierarchy of oppressions, and allocating 'truth' to someone who (in fantasy) occupies this 'politically pure' position. It never works. It almost always just reintroduces pre-existing hierarchies and acts to shut a lot of women up - and shut a lot of women out. 
Anyway, that aside, I'm finding this quite amusing/annoying. Ageism is incredibly powerful and destructive, isn't it?
In fact, she's not actually trying to shut older women up, per se, is she? She's not, technically, that young herself. What she's doing is trying to shut up/shut out a political opinion - and she's resolved that the best way to do that is to synonymise it with older women.
How fucked up is that? And how effective is that? It implicitly recognises that, while we have something we can call patriarchy, older women are considered, and treated, as something less than full political subjects. And their voices/actions/presence can be dismissed - without argument.
That is why she is aiming at silencing a political position by tying it to older women. It's easier to call for the silencing of older women than it is to call for the silencing of a particular political opinion.
It reminds me, powerfully, of an incident I encountered as a young woman. I worked on a feminist fanzine called Shocking Pink. We had a cut-off age of the production collective was 25 (the aim of the fanzine was to empower young women to produce magazines - so there was a justification to it) - although we welcomed contributions from women of all ages.
At one point, we were reviewed in The New Statesman, as part of an article/review of feminist magazines generally.
The review was fascinating: the reviewer simply didn't believe we were 'young women' at all. The general trend, at that time, was towards what was then dubbed 'post-feminism'. Apparently, 'young feminists' now spearheaded a feminism that was make-up friendly, 'sex-positive', 'porn-positive', and all about crashing through that glass ceiling ...
(Sounds familiar, yes?)
Well, we weren't. We were, as a collective, somewhat critical of all that.
However, instead of letting a small fact - like our actual age - get in the way of a prevailing discourse about what 'young feminists' espoused, and 'what this told us' about the present and future trajectory of feminism, the reviewer simply dismissed us as 'older women'. Really. The review said something like 'I think it is clear that we can see the hand of much older women guiding the views and writing in Shocking Pink.' This, of course, was utter bollocks. But how effective.
It's depressing. I'll say it again and again and again - if you don't espouse a popular opinion, you will find it bloody hard to get heard, whether you are old or young.
There is still an enormous amount of discrimination against women and ageism acts to add weight to that: when you are younger, you tend to have fewer contacts, funds, even experience. You tend to get dismissed on those grounds as a woman. When you get older, and perhaps have more experience, contacts, even expertise and research behind you, an anti-women ageism kicks in.
In both cases, the sole purpose of ageism is to strip you of the assets you may have, and your access to a voice - and power.
It is ridiculously anti-women to call on the tools of ageism to argue a political point against other women.