You may be shocked at how many adults dress in the image on the left (or maybe you wouldn't. Let me know in the comments). Sure, the 30+ year olds wearing the left might not have a backpack and don't wear the alien necklace, but if anything, I think they're worse off for it because I don't see them replacing it with another piece of jewelry- they simply don't accessorize. I feel it's better to try for a look and miss the mark than not try at all.

In your teens and into your 20s, box identity dressing is very popular. Hippie, punk, goth, nu goth, preppy, grunge, and so on are all marketed to you. If anything, it's quite difficult to dress outside of one of those identity boxes at that age, because it's what the youth stores sell. Sometime in-between your 20s and 30s, the box identities slow down and eventually stop. You get to the end of the youth market road and all of a sudden it's just tops, bottoms, and dresses and you're left to apply your own identity to it without having built that skill set.

Box identity dressing is really useful. It's a quick way of figuring out what you might want to buy, as well as what is likely to go with what you already own in a cohesive outfit. Style systems (Fantastical Beauty and Base 5) are more nuanced ways of narrowing the scope and creating an inspired identity for yourself. We can still use this approach, but we have to learn how to put it together for ourselves as it stops being fed to us.

The other thing that happens in your late 20s to early 30s is that showing a lot of skin goes from being fashionable to unfashionable. Suddenly you are supposed to be selective about the skin that you show. You're also expected to be making more money and choosing items of greater quality. You may find me running naked through the woods, so when I say these things, this is not a value judgement of mine, but an acknowledgement of external judgements and trends that we as individuals get to choose to adapt to or ignore.

My recommendation is to find your best personal style as soon as you're able, whether that is at 17 or 70, and begin building a wardrobe that will last you through the decades and work for any age. That brings us to question: what works for any age? Many things that look cute on a teenager, look twee on an adult, and many things that look fabulous on an adult, look staid on a teen. There are small tweaks that can be made as you age to transition a wardrobe (primarily skin and quality), rather than finding ageless pieces, but let's focus on striking an ageless balance with the following guidelines.

---8 guidelines For Ageless Style---

Limit your cotton tees and tanks without completely eliminating them, and focus most of your budget for tops on quality blouses and sweaters. Having a few cotton tees and tanks is fine, but unless they are part of your personal signature style, you don't need a drawer full, and not much should be spent on them. It's important that we don't stock up on our non-best items, because if they're around, they are what we will reach for. Better to have a pile of flattering blouses and sweaters. You could try a silk tee or one in linen, which are like blouses in disguise. Having one funky graphic tee is good for all ages and can add a valuable note of irreverence.
Keep a mix of pants and jeans, and keep mostly to non-trendy jeans to improve versatility. The hot pink distressed low rise jeans have a far more limited window than regular ole blue jeans. This is where knowing your personal coloring and FB5 types can be helpful. Plain ole blue jeans still come with a wealth of leg shapes, rises, lengths, and design detail options.
Dresses and skirts you could wear to a nice dinner. The studded pleather mini skirt might be worth having in your closet, but you won't get as many years out of it as, say, a pleather pencil or midi skirt. There are low cost ways to add more youth/funk/whatever you're trying to add. Invest in base pieces that will stay with you, and add your whims in cheap additive ways.
Cool flats will always be wearable shoes. Those are the ones to spend on.
Instead of showing skin with crop tops and low rise jeans, look to highlight the skin of your arms, back, shoulders, ankles, and calves (Again, I'm not pushing a modesty agenda. This is simply what I've noticed in terms of ageless style).
High fashion layering (not hap hazardous). Sometimes when an outfit is missing something, that something is a fabulous coat or jacket. This is a piece that can instantly make an outfit look really stylish or really sloppy.
Quality accessories will fit no matter changes in weight and age. Work on building a base of interesting and high quality accessories. Then you can play with cheap trendy pieces or flavor of the week styling. Think of jewelry, hats, scarves, purses, socks, and so on.
Remember KLM Addition