Neurobiology of Aging 30 (2009) 521–524 Open peer commentary Challenging the notion of an early-onset of cognitive decline Lars-Göran Nilsson a,d,e,f,* , Ola Sternäng a,d , Michael Rönnlund b,g , Lars Nyberg c,d,e,f a Department of Psychology, Stockholm University, Sweden b Department of Psychology, Umeå University, Sweden c Department of Radiation Sciences (Diagnostic Radiology) and Integrative Medical Biology (Physiology Section), Umeå University, Sweden d Stockholm Brain Institute, Umeå University, Sweden e Nordic Center of Excellence, Umeå University, Sweden f Aging and Living Conditions, Umeå University, Sweden g Center for Population Studies, Umeå University, Sweden Received 20 October 2008; accepted 6 November 2008 Abstract Salthouse claims that cognitive aging starts around 20 years of age. The basis for this claim is cross-sectional data. He dismisses longitudinal data, which typically show the cognitive decline to start much later, around 60 years of age. He states that longitudinal data cannot be trusted because they are flawed. There is a confounding between the effects of maturation and retest effects. We challenge Salthouse’s strong claim on four accounts. © 2009 Elsevier Inc. All rights reserved. Keywords: Cognition; Cross-sectional design; Longitudinal design; Neurobiology The present paper challenges Salthouseˇ ıs strong claim about an early cognitive decline on the following four grounds. First, development from childhood to mature age is a matter of individual development and cognitive change through different passages throughout the life span. Conceptually, we believe, statements about cognitive decline are highly related to cognitive change as seen in within- person comparisons. Second, both cross-sectional and longitudinal data are flawed if proper control is not applied. For cross-sectional data there is always a risk of a confound between true age effects and cohort effects. For longitudinal data, there is always a risk that retest effects are confounded with the true age effects. Various means have been suggested to control for cohort effects in cross-sectional data and for retest effects in longitudinal data. Third, Salthouse uses selective evidence from neurobiology and animal studies * Corresponding author at: Department of Psychology, Stockholm Uni- versity, SE-10691 Stockholm, Sweden. Tel.: +46 8 163940; fax: +46 8 159342. E-mail address: lgn@psychology.su.se (L.-G. Nilsson). to support his claim of this early onset of cognitive decline. Fourth, Salthouse is talking about cognitive functioning as if this is a unitary entity. We will deal with these four issues in turn. 1. Conceptually about cognitive decline It seems obvious that cognitive decline, in whatever age, is a matter of cognitive change. It seems self-evident to claim that within-person comparisons (as in longitudinal data) are more adequate to make than between-person comparisons (as in all cross-sectional data) when making claims about cognitive change of an individual. Salthouse seems to be of the same opinion as he writes in the last paragraph of the paper, “... only with longitudinal data can one examine within-individual changes distinct from between-person dif- ferences”. In light of this citation, the overall claim of the paper that cross-sectional data are to be preferred is intel- lectually confusing. The citation begs the question at the conceptual level of why the within-individual comparisons 0197-4580/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.neurobiolaging.2008.11.013