With length, [when you're younger] you can get away with having it extremely long. As you age, you can keep the long hair, but the proportion should be right for the time.
Everything that completely puts together the right look for you should be a factor. The color is very important. The older you get, you have to maintain the look. Don't hope at 40 or 50 you can have the same look you would have worn at 20. You can have the same idea, but the proportion should be a little bit different.
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