It's because before 40 years of age ,breast tissue is  more glandular ,hence usg is better option After that it becomes more of adipose tissue,where mammogram is better to assess any lesions At 30 it's sonomammogram or usg breasts is the best choice

The sensitivity ( ability to detect any significant lesion ) of mammogram in age less than 40 as breast is more glandular during that period Later it has more adipose tissue and hence the sensitivity increases Less than 40 it's better to undergo sonomamography Though that too is required only when some doubtful lesion is felt on breast self examination

Before 30 yrs it's sono mammogram