Network Aging Research, Heidelberg University, Bergheimer Strasse 20, 69115, Heidelberg, Germany. Department of Clinical Gerontology and Geriatric Rehabilitation, Robert Bosch Hospital, Stuttgart, Germany. Department of Health Economics and Health Services Research, University Medical Center Hamburg-Eppendorf, Hamburg, Germany. Institute of Epidemiology and Medical Biometry, Ulm University, Ulm, Germany. IB University of Health and Social Sciences, Study Centre Stuttgart, Stuttgart, Germany. Faculty of Health Sciences, University of Sydney, Sydney, Australia. Network Aging Research, Heidelberg University, Bergheimer Strasse 20, 69115, Heidelberg, Germany. schwenk@nar.uni-heidelberg.de.