Assigning Final variables from constructors | not allowed from methods. Why?

Practicing some code samples and i have came across this: I have declared final variables at class level and trying to assign values from methods, lead to compile time error(code-2). But from constructor it did get assigned(code-1).

code-1:

class Immutable {

    private final int age;
    private final String name;

    private Immutable(int age, String name) {
        this.age = age;
        this.name = name;
    }

code-2:

class Immutable {

    private final int age;
    private final String name;

    private void me() {
        this.age = 19;
        this.name = "name";
    }

Of-course, they could be assigned at class level too but, not allowed to do in constructors again, since they are allowed to declare only once. But, why are final variables assigned in constructors and not from methods?