In accordance with her figure and age. Older women can't wear outfits designed for, and predominately worn by young girls. Young girls have their brand of fashion that accentuates their youth and creates a sense of "togetherness of a generation". Clothes are a mark of social belonging. Wearing stuff young girls do, makes an older lady look like she's trying to fit in where she doesn't belong and that looks very awkward.
Second thing is figure. I know women over 40 who look like they are in their late twenties. Women who take care of their body and look GOOD. It would be ridiculous to expect such a lady to wear baggy, dark, unsightly clothing like her wrinkled, whale-sized age mates, just because she shares their age. It would also be silly for a woman (of any age) to wear clothes that don't flatter her figure, just because she likes the clothes. That's often plain ugly.
A woman should keep class, conservativism, body shape and simplicity in mind as she ages. With that she can't go wrong. And if nothing, she should get someone more fashion aware to help her pick out clothing.
According to one study a woman has over 400 looks in her lifetime. This was partly due to fashion trends and these days we don't have as many trends going on. So there's little choices. She should dress to her personality. If her personality is fun and sexy, then so be it.
43. Interesting age choice to ask about. Not 42. Not 44. Anyway, I'm a few years away from this age and I wear normal clothes. Jeans, blouse, t-shirt and yoga pants, dress pants with camisole and blazer - just depends on where I'm going or what I'm doing. I won't wear teenager-type stuff that is normally found in accessory stores. I like wearing real gold and diamonds if I wear jewellery, and if not, I'll only wear one item to accessorize. Not a lot of print, but if I do, it's subtle. Just because of my complexion, I look better in dark colours, but I do also have a love for red. I usually am told I look nice with my choice of clothes. My clothes are clean and they fit me, and I dress properly for my height and size. Same rule of thumb for pretty much all women I would think.
I discovered I was shortchanging myself by "dressing my age.". I had a period of time where I bought baggier shirts and mom jeans. Then, I discovered that I look much better in clothes that actually fit me. Now I wear my size 6-8 jeans, medium T shirts, mid thigh length skirts. I looked HUGE in baggy clothes! Wearing things that fit makes me look slim and makes me feel fantastic.
I can judge a pattern or style of something and and can file it as " waaay too young. ". At the same time, I don't hide under bland or bulky stuff anymore. There is a balance. The key is to go for a flattering look vs " too young" or "too old.". Dress for your figure and you can't go wrong.