diff --git a/include/system/window.h b/include/system/window.h
index a87542758..508ce00ba 100644
--- a/include/system/window.h
+++ b/include/system/window.h
@@ -267,7 +267,16 @@ enum {
      * The default data space for the buffers as set by the consumer.
      * The values are defined in graphics.h.
      */
-    NATIVE_WINDOW_DEFAULT_DATASPACE = 12
+    NATIVE_WINDOW_DEFAULT_DATASPACE = 12,
+
+    /*
+     * Returns the age of the contents of the most recently dequeued buffer as
+     * the number of frames that have elapsed since it was last queued. For
+     * example, if the window is double-buffered, the age of any given buffer in
+     * steady state will be 2. If the dequeued buffer has never been queued, its
+     * age will be 0.
+     */
+    NATIVE_WINDOW_BUFFER_AGE = 13,
 };
 
 /* Valid operations for the (*perform)() hook.