This should help you out. I've created a textwatcher and the simple method inputValidation which returns a boolean whether the editable is empty or not. If the edittext is blank when the validation label is displayed. 
TextView input_validation;
EditText input;
int final_input_value;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    setupViews();
}
public void setupViews() {
    input_validation = (TextView) findViewById(R.id.input_validation);
    input_validation.setVisibility(View.GONE);
    input = (EditText) findViewById(R.id.input);
    input.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence charSequence, int i, int i2, int i3) {
        }
        @Override
        public void onTextChanged(CharSequence charSequence, int i, int i2, int i3) {
        }
        @Override
        public void afterTextChanged(Editable editable) {
            if (inputValidation(editable)) {
                input_validation.setVisibility(View.VISIBLE);
                input_validation.setText("You must enter your Age");
            } else {
                input_validation.setVisibility(View.GONE);
                final_input_value = Integer.parseInt(editable.toString());
            }
        }
    });
}
public boolean inputValidation(Editable inputText) {
    return inputText.toString().isEmpty();
}
Layout
  <TextView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/input_label"
    android:textAppearance="?android:attr/textAppearanceLarge"
    android:text="Your Age"
    android:textStyle="bold"
    android:textColor="@android:color/holo_blue_dark"
    android:paddingBottom="16dp" />
<EditText
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/input"
    android:inputType="number"
    android:layout_below="@+id/input_label"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />
<TextView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/input_validation"
    android:textStyle="italic"
    android:textColor="@android:color/holo_red_dark"
    android:layout_below="@+id/input"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />
The output looks like this.
