How to change items in a list when iterating through the list.

How can I iterate through a C# list, and change the values in the list. Below is some code to refer to. Basically in this example I want to change everyones age to 25.

  public class Person
    {
        public string Name
        {
            get { return Name; }
            set { Name = value; }
        }


        public int Age
        {
            get { return Age; }
            set { Age = value; }
        }
    }

    public partial class Form1 : Form
    {

        List<Person> Customers;
        Person newPerson = new Person();
 

        public Form1()
        {
            InitializeComponent();

   
            newPerson.Name = "Bob";
            newPerson.Age = 30;
            Customers.Add(newPerson);

            newPerson.Name = "Mickey";
            newPerson.Age = 29;
            Customers.Add(newPerson);

            newPerson.Name = "Jan";
            newPerson.Age = 29;
            Customers.Add(newPerson);
           
            foreach(Person p in newPerson)
            {
               p.Age=25;
            }
        }