Old-age employment and hours of work trends: empirical analysis for four European countries / Arjeta Aliaj (HEC, University of Liege), Xavier Flawinne (HEC, University of Liege), Alain Jousten (HEC, University of Liege and IZA), Sergio Perelman (HEC, University of Liege), Lin Shi (HEC, University of Liege) ; IZA