As Kintaro already says, person is a method (because of def) and always returns a new Person instance. As you found out it would work if you change the method to a var or val:
val person = new Person("Kumar",12)
Another possibility would be:
def person = new Person("Kumar",12)
val p = person
p.age=20
println(p.age)
However, person.age=20 in your code is allowed, as you get back a Person instance from the person method, and on this instance you are allowed to change the value of a var. The problem is, that after that line you have no more reference to that instance (as every call to person will produce a new instance).
This is nothing special, you would have exactly the same behavior in Java:
class Person{
public int age;
private String name;
public Person(String name; int age) {
this.name = name;
this.age = age;
}
public String name(){ return name; }
}
public Person person() {
return new Person("Kumar", 12);
}
person().age = 20;
System.out.println(person().age); //--> 12