Just replace this function

import java.util.Scanner;

public class test {

private static int number1 = 100;
private static int number2 = 1;

public static double avgAge() {

    return (number1 + number2) / 2.0;
}

public static void main(String[] args) {

    System.out.println("Average number: " + test.avgAge()); //Average number: 50.5
}   
}

First off you need to understand the difference between the two types. double is a primitive type whereas Double is an Object.

The code below shows an overloaded method, which I assume is similar to your lab code.

void doStuff(Double d){ System.out.println("Object call"); }
void doStuff(double d){ System.out.println("Primitive call"); }

There are several ways you can call these methods:

doStuff(100);
doStuff(200d);
doStuff(new Double(100));

These calls will result in:

"Primitive call"
"Primitive call"
"Object call"