When you can get signed as a fashion model when under 5'9"
In certain markets, some agencies will sign you if you are 5'8".
If you are young and still growing, and the agency forecasts you to grow to their minimum height, they may sign you (see more in Teen Models).
That's it. If you do not fall into one of these categories, it's pretty much absolute. You will need to consider another genre of modeling (See "The Genres of Modeling" in the blog and read Get SIgned in the Model Portal).