Show additional field based on a Drop Down selection

I have a volunteer form.

We need to ask if they person in under age 18.  I have a drop down shown in the image provided "under 18? y/n"

If they choose yes, how do I get another box to pop up so that the person can type in their age?

It is possible to use conditional loginc on form to do this:

Smart Forms Using Conditional Logic

Here is a quck screencast:

http://screencast.com/t/KlQSmCC7V

If you need any further assistance, please let us know. We will be happy to help.