Parsing JSON Array within JSON Object

I have some JSON with the following structure:

{"source":[
           {"name":"john","age":20},
           {"name":"michael","age":25},
           {"name":"sara", "age":23}
         ]
}

I have named this JSON string as mainJSON. I'm trying to access the elements "name" and "age" with the following Java code:

JSONArray jsonMainArr = new JSONArray(mainJSON.getJSONArray("source"));
for (int i = 0; i < jsonMainArr.length(); i++) {  // **line 2**
     JSONObject childJSONObject = jsonMainArr.getJSONObject(i);
     String name = childJSONObject.getString("name");
     int age     = childJSONObject.getInt("age");
}

I'm being shown the following exception for line number 2:

org.json.JSONException: JSONArray initial value should be a string or collection or array.

Please guide me as to where I'm making the mistake and how to rectify this.