ESB WSO2 Java - I want to modify a value from a json file with Class Mediator

So I have this Class Mediator for my API:

public class CheckUser extends AbstractMediator {

    public boolean mediate(MessageContext context) {
        String age_value = (String) context.getProperty("age_value");
        context.setProperty("age_value", 33);
        log.info("#### The new age value is: " + context.getProperty("age_value"));

        return true;
    }
}

And I want to modify the age value in the response

Initial json file:

{
    "data": {
        "user": {
            "name": "Andrei",
            "adress": "Tudor Vladimirescu, nr2",
            "cnp": 123456789,
            "age": 22,
            "valid": "Yes"
        }
    }
}

Desired json file:

{
    "data": {
        "user": {
            "name": "Andrei",
            "adress": "Tudor Vladimirescu, nr2",
            "cnp": 123456789,
            "age": 33,
            "valid": "Yes"
        }
    }
}

Thank you!