At the time this was written, MRJ (Apple's runtime environment for Java) does not support the Java 2 standard. The MetroWorks compiler will compile Java 2 code, but it will generate an error when you attempt to run it on the Mac.
The method parseDouble() is therefore not supported. You can accomplish the same thing by replacing:
double age = Double.parseDouble(ageString);
with
double age = Double.valueOf(ageString).doubleValue();
The swing package is supported by MRJ. To get it to work you will need to add the library swingall.jar. It should be in the directory:
CodeWarrior Pro 5:Metroworks CodeWarrior:Java Support:Libraries.
Make sure your project is open in IDE and click and drag swingall.jar to your project window (the XXXX.mcp window, where XXXX is the name of your project.).