For the determination of ages of OB associations, the traditional approach is to compare isochrones, derived from theoretical models to the observed distribution of stars in the HR diagram (see e.g. Stothers, 1972). However, even a casual look at the HR diagram of the association Per OB1 (Fig.1) shows that it is impossible to fit a unique isochrone to all stars of the group, even by allowing some scatter. We will therefore adopt a different approach and determine the initial mass and the age of each individual star in a given association. As an example, we will discuss Per OB1.