How to assign new attributes into spark Dataset in Java

Assuming I have a Dataset<Person> personList, that contains list of Person.

Person is defined as follows:

public class Person {
    String name;
    String gender;
}

Now I have the list personList as dataset, but I need to backfill another attribute into Person, let's say it's age. So I can update my Person to

public class Person {
    String name;
    String gender;
    int age;
}

How do I loop through the Dataset and upate the age value?

I tried this approach, but it didn't update anything:

    personList.foreach(person -> {
        person.setAge(12);
    });

I tried to give every Person in the personList age of 12, but when I read the data set, the age value is still empty.

Why?