In case somebody want to know.

The values in the ATAG atlas reflect the number of people of the sample that share a voxel for a given structure. If you want to normalize the values between 0 and 1 you will have to divide the masks with the respective maximum value possible for each age group. For the young subjects we included 30 subjects, so the max value is 30, for the middle-aged group the max value is 14 and finally for the elderly group we included 9 participants so the max value is 9.