The early events of T-cell generation, i.e. seeding of bone marrow-derived progenitor cells onto the thymic stroma, involve a small fraction up to 1% of thymus cells and are not presently observable. However, these events are crucial in determining the outcome of thymic colonization. In previous-studies we utilized an experimental in vitro model of thymic reconstitution by bone marrow cells to compare normal thymocyte development with the development in conditions of T-cell deficiency, and, in particular, in aging. These studies showed that progenitor cells from old donor bone marrow are deficient in their ability to colonize the thymus, in spite of their ability to divide earlier upon seeding. In this study we apply mathematical and computer modelling in order to analyse early T-cell development and the causes for the developmental disadvantage of old donor bone marrow cells. The results indicate that the competition for seeding niches in the thymic stroma determines the outcome of colonization.