Computational thinking is a fundamental competence that is being introduced in K-12 and succeeding curricula worldwide. Despite this huge effort, many computational thinking models in the literature do not explicitly take into consideration the pupils’ age and the developmental nature of computational thinking skills. Furthermore, many existing computational thinking models are focused on the internal thinking processes of the individuals, failing to explicitly consider the situated nature, in terms of social context and artefactual environment, that usually characterise tasks that require computational thinking to be solved. In this paper, we present a framework for the design, realisation, analysis, and assessment of computational thinking activities, called CT-cube. The CT-cube allows to extend existing computational thinking models to consider the life-long development of computational thinking skills in individuals, from childhood to adult age, and to take into consideration the situated nature of computational thinking activities. We use the CT-cube to design an unplugged task, called Cross Array Task (CAT), allowing to assess the algorithmic skills of K-12 pupils and we show how the CT-cube can be used in this case to illustrate the development of this competence along the entire compulsory school path, considering a sample of 109 pupil aged 3 to 16 in Switzerland.