How do I group a set of radio buttons with a nice border and title?

You can use a Swing Border to draw a decorative titled border around the group of radio buttons.

The main "trick" here is to place the buttons you want to group in a JPanel by themselves, then add a Border to the JPanel.

The following code shows a simple example of doing this.

import javax.swing.BorderFactory;
import javax.swing.ButtonGroup;
import javax.swing.JFrame;
import javax.swing.JPanel;

import javax.swing.JRadioButton;
import java.awt.FlowLayout;
import java.awt.GridLayout;

public class Sample2 {

  public static void main(String[] args) {
    new Sample2().go();
  }

  public void go() {
    JFrame f = new JFrame("Sample");
    JPanel p = new JPanel(new GridLayout(0,1,3,3));
    p.setBorder(BorderFactory.createTitledBorder("Age group"));
    JRadioButton b1 = new JRadioButton("< 6");
    JRadioButton b2 = new JRadioButton("7-13");
    JRadioButton b3 = new JRadioButton("14-20");
    JRadioButton b4 = new JRadioButton("21-35");
    JRadioButton b5 = new JRadioButton("36-62");
    JRadioButton b6 = new JRadioButton("> 62");

    p.add(b1);
    p.add(b2);
    p.add(b3);
    p.add(b4);
    p.add(b5);
    p.add(b6);
    
    ButtonGroup group = new ButtonGroup();
    group.add(b1);
    group.add(b2);
    group.add(b3);
    group.add(b4);
    group.add(b5);
    group.add(b6);
    
    f.getContentPane().setLayout(new FlowLayout());
    f.getContentPane().add(p);
    f.pack();
    f.setVisible(true);
  }
}