This medical condition can occur at three points during the life span of an individual. The first occurrence is when we are babies, the second at adolescent stage during puberty and finally in late adulthood after 55 years. Gynecomastia during puberty is called pubertal gynecomastia or teenage gynecomastia and is usually temporary. The breast tissue developed during puberty while boys go through adolescence should even out once the chest flattens and boys enter adulthood.The excess breast tissue that develop in some men during their adolescence stays  even as they enter adulthood and this is when it becomes a genuine and confirmed case of gynecomastia. This is not normal process because the excess breast tissue should have flattened out by this time. In a nutshell, if you are in adulthood and have excess breast tissue, then it is gynecomastia and will not go away naturally.
Some other causes of gynaecomastia needs to be mentioned as medicine like estrogen, diseases like cirrhosis, hypothyroidism etc.