How to set value for a dummy question by javascript

I have an "Age" question, with type number like this:

Then I want to convert these number into some groups of age, such as:

How to do by javascript?

- First, create a single question "D_AgeGroup" right after "Age" question, with such options:

- Second, create a page question type multiple questions per page:

- And then add "Age", "D_AgeGroup" to that multi page:

- After that, at "Age" question, go to CSS tab to hide "D_AgeGroup"

- Finally, also at "Age" question go to Javascript tab, write script for setting value of "D_AgeGroup" from "Age"

You are done.

Viewer only show question of Age. When inputing value on Age question, the approriate value will also set to D_AgeGroup: