This work aimed to (i) expand the dataset on gastric fluid composition in the paediatric population (0–18 years old) and (ii) improve our understanding of age-dependent changes in gastric fluid characteristics involved in gastrointestinal drug disposition. For this purpose, gastric fluids from preterm neonates, term neonates, infants, children and adolescents were collected during routine medical procedures. Gastric fluid constituents relevant for gastrointestinal drug disposition were characterized i.e., pH, osmolality and bile salts (concentration + composition). Differences in gastric fluid composition compared to adults were most prominent in neonates. In this context, the fact that neonates are rarely fasted due to frequent feedings should be taken into account during paediatric drug product development. It remains to be explored to what extent the observed variability and differences in gastric fluid characteristics within and between age groups translates to variability and/or differences in oral drug disposition.