Does atrophy of the thymus not effect T-cell selection and tolerance?
T cells have a long half life (both naive and memory cells have halflives measured in years), especially when taking into account the fact that memory cells in particular can slowly proliferate and renew themselves.  An adult human therefore has 20-50 years' worth of accumulated T cells by the time they grow old, and those include both memory cells and naive cells that are potentially able to recognize new antigens. The thymus does continue to produce some new cells at a slow rate, but the backlog of previously produced cells are also able to monitor for pathogens.