This chapter describes a study about how students from different age groups, education levels and programmes, answer PISA items and how they evaluate different aspects of the items (e.g. “understanding” the question and assessing its “difficulty”). We administered a booklet with two sets of publicly available Mathematics items (from PISA 2012) and Science items (from PISA 2015) to a non-representative sample of 839 Portuguese students from Basic, Secondary and Higher Education (approximately 50% are over 18 years old), from different programmes and schools, public or private. Comparing the scores on our sample and on the same items in PISA tests, for both Portugal and OECD countries (students aged 15), we found similar results across all ages groups of our sample, in both domains. This suggests that PISA tests can target older students and that knowledge and skills at age 15 and older ages are globally similar (at least in Mathematics and Science). We also found that item facility has a significant positive correlation with students’ assessment of the comprehension of the items, with the certainty of the answer, and with the assessment of the item difficulty and has no significant correlation with the student study of the item content.