Artículo de publicación ISIPopulation dynamics and their response to environmental stressors have been widely studied in intertidal organisms. However, how these dynamics and responses change with animal age have been largely ignored to date. Traditionally, it is assumed that younger organisms are more sensitive than adults to environmental stressors; under this perspective it could be predicted that fully grown organisms should be able to occupy the harsh upper limit of their intertidal habitat. However, in some intertidal Porcelain crabs the opposite distribution has been observed. Using Petrolisthes laevigatus, we tested the physiological tolerance of crabs of different sizes (i.e. age) and evaluated how this trait shapes population dynamics (distribution and small-scale migrations under different weather conditions). We determined the abundance and size distribution of P. laevigatus at the middle and upper intertidal levels during sunny and rainy days, finding that abundances decreased drastically and size distribution shifted to smaller individuals on rainy days. In the laboratory, survival and behavioural responses of individuals in water at 5, 10, 15 and 33 PSU salinities were evaluated. Young crabs were found in higher proportion in the upper intertidal while fully grown crabs (i.e. adults) mainly occupied the middle intertidal zone. Young crabs had a higher osmoregulatory capacity than adults, as they were better at regulating passive water uptake when challenged with diluted seawater. This was also correlated with a lower lethal salinity LC50 in young crabs compared to adults. Behavioural trials showed that young crabs performed better escaping in both water and air, at intermediate and reduced salinities than adults. Therefore, weather influences small scale migrations from the upper to the lower intertidal zone, and this migration is also age-dependent, with younger crabs being more tolerant to low salinities and therefore allowing them to remain in the upper intertidal zone during raniny days.