Find out what you can get away with in your age group
Your 20's: You can try out all kinds of cuts and styles, many opt for the romantic and pretty long hair styles, but others either want to look younger or older so they get shorter haircuts (whether you look younger or older really depends on the style and the competency of your hairdresser). Beware of getting short cuts that are not ideal for your face shape and age, you don't want to end up looking like a cartoon with a boyish mushroom cut. Highlights and lowlights are always a great way to achieve dimension with any haircut, so consider getting them in a tone matching the full color of your hair.
Make sure to always choose a look that fits your age and latest trends. It's important to consider your career since you are most likely establishing what it will in your 20's. If you're more into the arts you can probably get away with some funky colors and fun styles, but if you're in a conservative profession you should steer clear of anything that might be construed as rebellious.
You should experiment with length and color because this is the time that you can, later on maybe your hair won't grow as beautifully long and your face may not lend itself to trying all sorts of dye colors. In your 20's your hair will grow faster and thicker than any other time in your life, so make sure you take care of it and take advantage of it!
Don't be afraid to try new haircuts or go short with your hair, some people just naturally look better with short hair, especially if you have beautiful facial features, they will be highly accentuated without all that hair in the way. If you're starting to have significant amounts of gray hair you can start coloring your hair pretty regularly, or you can ask your stylist to hand-paint the couple of strands of gray that you have in a tone that matches your hair, so that you don't have to color fully. Another great way to camouflage gray hair is to color it with a color of mascara that matches your natural hair (they make mascara in tons of shades now).