Why does gynecomastia happen only to males at certain age groups?

The bottom line with gynecomastia is that it occurs due to an abnormality in the ratio of testosterone to estrogens in the body. When this ratio is low, the estrogen effect is stronger and stimulates the growth of the tissue around the breast. The testosterone which is most important is that which is not bound to protein in the blood, in other words, the free serum testosterone. This has been found to be lower in boys with gynecomastia compared to those without, while all the other hormone levels were about the same.