Should members of static class in Builder pattern be final?

So I have seen a couple of examples on Builder Pattern. In all, The static class or Builder only has those memebers as final that can be intitialized by the constructor. Example -

public class User {
        private final String firstName; // required
        private final String lastName; // required
        private final int age; // optional

    private User(UserBuilder builder) {
            this.firstName = builder.firstName;
            this.lastName = builder.lastName;
            this.age = builder.age;
            this.phone = builder.phone;
            this.address = builder.address;
        }

    public String getFirstName() {
            return firstName;
        }

    public String getLastName() {
            return lastName;
        }

    public int getAge() {
            return age;
        }

    public static class UserBuilder {
            private final String firstName;
            private final String lastName;
            private int age;

    public UserBuilder(String firstName, String lastName) {
                this.firstName = firstName;
                this.lastName = lastName;
            }

    public UserBuilder age(int age) {
                this.age = age;
                return this;
            }
         }
    }

Why is that only First and Last Name are final that can be intitialized by the constructor? And it cannot be set after that? My doubt is Can't I remove the final from First and Last and have a constructor which initializes all three members and have corresponding setters too in the class?