You just need to index the property in ES (like any other property).
Indexed in Titan (wrong; orderBy will not be optimized):
graph.makeKey("age").dataType(Integer.class).indexed(Vertex.class).make();
Indexed in ES (correct; native result ordering in ES):
graph.makeKey("age").dataType(Integer.class).indexed("search", Vertex.class).make();
Cheeers, Daniel