Correct identification and explanation of age-related changes are always possible.

In this commentary, it is argued that the definition of the time or the age variable in the models presented by Hofer and Sliwinski is ambiguous, and a standard interpretation of this time of age variable is proposed instead. Some simulation studies are presented, the results of which indicate that, even with age-heterogeneous cross-sectional data, correct identification of the true sources of age-dependent variation is possible. An important qualification, based on new methodological work in progress, is given of the scope of longitudinal designs. It is concluded that all kinds of age-related changes are potentially interesting and can be correctly identified and explained. Copyright 2002 S. Karger AG, Basel