Can you explain this better? You need something finds the interval of ages
of someone in a JComboBox?

This implementation is easy. Use String arrays to build your combobox.

String[] values = new String[3]; // I am not 100% sure with this syntax. Just
check it
values[0] = "15-25"
values[1] = "26-30"

JComboBox age = new JComboBox[values];

Use this combobox  in your program by adding it to your panel.

To retrieve the information entered by the user, get the selected index value.
using a simple switch statement or a if-else u can retrieve the values.