Autism spectrum conditions (ASC) develop early in life and are thought to last a lifetime. However, ASC research has two major knowledge gaps that hinder progression in understanding the concept of ASC and in providing proper support for autistic adults: 1) the majority of knowledge about ASC mainly stems from childhood studies so little is known about older autistic adults; 2) while it is broadly recognized that ASC is a heterogeneous condition, we do not yet understand the differences in trajectories leading to their future outcome. We aim to fill both knowledge gaps.A multistage overlapping cohorts design assessing (cognitive) aging in ASC, is designed to obtain an accelerated longitudinal dataset. Data, including a multitude of questionnaires, diagnostics, and cognitive tests, are collected over four waves within a 10-year time frame. This will provide information regarding actual changes in quality of life, co-occurring health conditions, and cognition as well as the possibility to test external validity and temporal stability in newly formed behavioral subtypes. Participants consist of three groups of adults aged 20 to 90 years: 1) with a clinical diagnosis of ASC; 2) with a clinical diagnosis of ADHD, but no ASC; 3) no ASC/ADHD (i.e., comparison group; COM). The sample size differs between waves and instruments. Detailed analysis plans will be preregistered in AsPredicted or at the Open Science Framework. Ethical approval for this study was obtained from the ethical review board of the Department of Psychology of the University of Amsterdam (Wave 1 2011-PN-1952 and 2013-PN-2668, Wave 2 2015-BC-4270, Wave 3 and 4 2018-BC-9285). In line with the funding policies of the grant organization funding this study, future papers will be published open access.