I don't think you can define middle age lljkk with regard to fashion. Some 40 yr olds look ancient, others nearing 60 or older look great- think Lulu, Twiggy, Felicity Kendal, Irman, Sandra Howard, etc etc.
Unless of course you mean in terms of life-expectancy. Then 40 is about right.
Every age group makes faux pas- from teenagers showing their muffin tops over their low rise jeans, to women of any age not covering up droppy tits , showing off flabby arms- age is not really the issue, it's what kind of shape you are in.
To be honest, I think the biggest agers are being overweight- nothing looks good really if you are carrying 2 stones too much- and having bad teeth ( stained or dark) and bad hair.
Bad hair equals all-over dyed helmet, too much bleach, dark roots, spikey short with gel, scappy hair that needs a cut, and grey ahir- sorry ladies I know this has been done to death, but very very few women can get away with grey hair- it has to be in great condition, beautifully cut, and an even colour- not salt and pepper or dark and light grey.
I think my pet hates are:
pearly lipsticks and yellowy teeth
blue eye shadow
elasticated waist trousers or trousers with pleats at waistband
calf length floral or patterned skirts
the wrong kind of shoes with the outfit- i.e. navy low heeled court shoes that go ( or rather don't) with everything.