There are a lot of different types of sleeves. They run the gamut from super puffy to non-existent. For the most part, sleeveless garments are not flattering. Even if you have beautifully sculpted arms, which most of us don't, garments without sleeves leave your skin showing and it will likely be high contrast to the rest of the outfit. Unless you're in the habit of wearing skin tones, this contrast chops up the figure and the flow.
There are also few people that should wear very puffy or full sleeves. These add a great deal of weight to the design. If you are very thin and have narrow shoulders, you might find a fuller sleeve beneficial. The rest of us should stay away from them.
The sleeve seam placement can be as important as the length and fullness. Some sleeves are attached off the shoulder. This gives the appearance of much broader shoulders. If you need this to balance your figure, its a good option. If you are trying to slim down the shoulder/bust area, stay away from the raglan and dolman sleeves.
The best sleeve design for most of us is a three-quarter length, with minimal fullness attached where the arm meets the shoulder line.
There are a lot of neckline styles. Not only are there different necklines, there are all kinds of neck accessories that can drastically change the style line. Scarves, necklaces and collars can all have an effect on the neckline.
For the most part, remember that more skin means more attention. If you want to draw attention away from your lower body, use a more open neckline. Again, this is the contrast theory. Your skin will contrast with the garment and draw attention.
Frills, ruffles, and high collars, like turtlenecks and cowls, shorten your neck. It is usually to your advantage to keep your neck looking as long as possible. It adds to your vertical height. There are some beautiful cowls and they can be complementary, just keep them loose and hanging down in front. When they are tight it creates another one of those bad horizontal lines. A loose flowing cowl will drape vertically.
Collars come in all sizes and widths. A narrow collar is usually your best choice. A boxy, square collar will fill out the chest area. If that is flattering to your body type, then keep that in mind.
Skirt & Pant Length
The length of your skirts and pants is important. Hemlines are one of the most drastic horizontal lines of a garment. The most flattering hem length for most skirts is just above or below the knee. Skirts that are longer can make you appear shorter and wider. Skirts that are short require an excellent pair of legs. Also, short skirts are not a good idea for someone who is short waisted, as their legs will seem much more out of proportion.
Pants come in all kinds of lengths and the best is at the ankle because it doesn't break up the vertical line. Bermuda, pedal pusher, toreador and capri pants all chop off your legs, from a design perspective. If you like these styles but benefit from a longer leg line, consider wearing a pair of tights that reduce the contrast. This isn't always practical, especially in the summer. Sometimes you have to compromise on style for comfort.
Short shorts are in the same boat with short skirts. If you have amazing legs and are not overweight, then go for it. Otherwise, you may want to consider something else.
Layering is a fashion trend. Personally, I think it's prompted by cheap clothing that is so thin, you have to wear more of it to cover yourself.
Layering done well can be very complementary. I don't think I've ever seen anyone look bad in a well tailored suit. Just remember to watch the lines that each subsequent layer adds to your overall garment.
Jackets, sweaters and vests will add bulk. If they have pockets, it can get even bulkier. Keep an eye on the belts and where the hemlines end up.
There's nothing wrong with a bit of layering, it's a great way to add temperature adjustments. Just don't get carried away with it and stay away from cheap clothes that require it.
In closing I would like to make a few comments on the subject of age. I don't think that there are clothes more appropriate for specific ages. Make your garment choices based on your body, your lifestyle and your attitude, not your age.
Gender is also something that doesn't define style. My grandfather wore a dress as a child and that was completely acceptable at the time (born 1905). Don't put yourself in a box. Have fun with it.
Remember that the most important part of your personal fashion is feeling good about yourself. All my examples and recommendations are offered only to allow you to get the most compliments you possibly can and boost your self-esteem. You deserve to feel good about your body, whatever size or shape it may be.