I'm with MaryK, Jonesy and Tracina on this. I'm uneasy with a bunch of shoulds and shouldn'ts about what to wear at a certain age. Apart from feminist concerns (which are important), it doesn't reflect the independence I like to express through fashion (or at least the feeling of independence). And it's illusory. Rules about fashion generally are fluid and not wearing something just because you think you're not "supposed" to, often leads to frustration, especially when you see someone else pulling off the look you'd just banned yourself from.
I'm fortunate in that every day I see women over 40 wearing a variety of looks, some of them boldly - or ironically - youthful. Some pull them off better than others, but I suspect they'd pull off an older look better than others too - it's about having a foundation of being in touch with yourself, your milieu, and of course, your style.
I love that Suz used the term "gravitas" because that's a word I've been mulling over lately. I think a fear-based approach of whether we are MDAL or not is less relevant than an approach where we ask ourselves how we can incorporate an element of gravitas, especially when we also choose playfulness (which is also grown-up - a much more intentional word than "fun").
And then for some of us who convey nothing but gravitas, perhaps choosing to convey some vulnerability is in order.
Being grown-up is about being multi-faceted, present in the now (not the forward-looking of youth nor the nostalgia of very old age), and enjoying the confidence we get through being in charge of our own lives.