I think it's more that women of a certain age simply look BETTER in certain styles than they do in others. People also say "She shouldn't be wearing that" about a very overweight 25-year-old in tiny shorts. I think that, at any age, there is an unwritten rule that we all should dress in what flatters our bodies and covers us appropriately, and to do otherwise would be considered distasteful or even obscene.
I think there's also the assumption that women in your age range are mothers, and so should dress in a way that is appropriate to be around children. Most people would feel uncomfortable at the sight of a kindergarten teacher in daisy dukes, bare tummy and massive cleavage, even if she has a perfect body.
No. These rules are actually unwritten societal rules tied to age stereotyping. Older people are supposed to be conservative, understated, and asexual. Why? Because they're old, out of shape, unhealthy and should be concerned with serious things - not frivolous things like fashion or looking good. Who are they trying to impress anyway? They can't impress anyone because everyone knows they're old and unattractive.
And if they are attractive, they should still tone it down because they had their turn and now it's time for young people to shine on stage. Any attempt by older people to signal through their clothing that they are not conservative, understated, or asexual is met with ridicule, pity, and accusations of "trying to recapture their youth" or "trying to compete" with their daughters (I had no idea you were competing with anyone when trying to look good and express your style).
If I had to put this in a nut shell I would say that typically older women are not seen as good looking or sexy - in fact they are regarded as the opposite - wrinkled, flabby, and gross to look at. For those who don't get that, they are told what to wear so they don't assault our collective senses with their gross giggly oldness.