Lowest income quartile is equal to 1 if the individual’s income falls in the bottom 25% of the national income distribution, it is equal to 0 if the individual’s income is higher than the lowest quartile. Age dummies are equal to 1 if the individual’s age falls in the age range of each dummy (0/1 variable). The age category 15 to 24 is the reference group with respect to which the results for the other age categories should be interpreted.