Issue With Printing Text to Command Line

Question

When parsing command line input using Scanner class in Java, there was an issue printing the last value. It can be fixed by changing the variable age to a String, but I would like to know the cause of the initial problem. The code is below.

import java.util.Scanner; 

public class Main {
    public static void main(String[] args) {
        
        // Create new instance of scanner
        
        Scanner scanner = new Scanner(System.in);
        
        // Ask for user details
        
        System.out.println("What is your first name?");
        String name = scanner.nextLine();
        
        System.out.println("What is your last name?");
        String surname = scanner.nextLine();
        
        System.out.println("How old are you?");
        int age = scanner.nextInt();
        
        System.out.println("Make a username.");
        String username = scanner.nextLine();
        
        System.out.println("What city do you live in?");
        String city = scanner.nextLine();
        
        System.out.println("What country is this city located in?\n");
        String country = scanner.nextLine();
        
        // print variable values
        
        System.out.println(name);
        System.out.println(surname);
        System.out.println(age);
        System.out.println(username);
        System.out.println(city);
        System.out.println(country);
        
        
    }
}

Thank you in advance.