The ambitious goals for the sustainable future of the global community seek to ensure that "all learners acquire the knowledge and skills needed to promote sustainable development." It is well known that questionnaire tests can serve as powerful incentives to prompt schools, teachers and students themselves to work towards centrally established education outcomes. Our efforts are devoted to the problem of youth sustainability competence assessment. The essential role of quantity is known for understanding the challenges at sustainability. Sustainable development is the subject of extreme complexity. Among the attempts to capture its main features and quantities, the technique of sustainability performance indicators is well recognized through the concept of Life Cycle Thinking. The life-cycle approach for sustainable development has been undertaken. It provides with well-based qualitative and quantitative outlook at sustainability challenges. Particularly, the assessment of youth sustainability competence has been undertaken towards continuous education system for sustainable development. The results of the study are concerned to the following themes: (a) the family of questionnaires to assess the sustainability competence, based on measuring knowledge, attitudes and behaviors, as well as indicators and monitoring framework for the sustainable development goals, have been tested; (b) a model outlook on the complexity of cognitive strategies based on the "transformation theory of learning" for adequate interpretation of the provided assessments, which are applicable to many kinds of questionnaires; (c) the "dissemination principle" has been reviewed as a sustainability implementation mechanism when the sustainability mode is adopted by the "main proportion" of the stakeholders being above some threshold level.