For the most part, women in their 40s shouldn't wear the same clothes as they did when they were in their 20s. This is particularly important for women to remember who have remained the same shape and size. Your body may look the same, but you've had so many more experiences with the world and the message that you want to put out there should shift in accordance. It's irresponsible for a mother to dress like her teenage daughter and I see it all the time! Same applies to men. I'm constantly surprised at how many men I see in their 50s dressing exactly the same way as they did when they were in high school.
That said, dressing age-appropriately does not have to mean being matronly or dowdy; it just means you shouldn't show too much skin. No matter your age, however, it's key to remember that getting it right has nothing to do with individual items, but rather with three intangible items — silhouette, proportion and fit. When those elements are in balance, you will look good no matter what you are wearing.
I will add though that there are certain things you can invest in, in your 20s, and they will last you decades to come. I'm a big believer in investing in outerwear. A trench coat, for example, is great because you can wear it to the grocery store just as easily as you could to meet Oprah and it doesn't have to be the classic Burberry khaki trench, it can be anything that resonates with you and your lifestyle.