If the model was created with Pyramid, you should use an independent column to determine the sort order since your captions are sorted using plain text.

First, create the complementary sort order column and add it to your database or bring it in from a secondary source in a mash-up.

In Model, under the 'data model' section, in the "columns" page, you can then use the second column to determine the sort order for this attribute as needed.

You could also name them 1. 0 - 30, 2. 31 - 60 etc. so it will automatically sort them 

Otherwise, right click on the blue attribute chip and use "Custom Order", then select the members in the order you want them to appear:

Hope that helps.

Ian