I've often heard of "balls dropping" occurring during infancy, but does this happen in a later Tanner stage as well.
When does the corona develop, during infancy?
Prior to birth, a boy's testicles form in the abdominal cavity. Just before birth (usually during week 28), they drop through holes in the abdomen into the scrotum. The descent might take a few weeks to complete, but normally by twelve weeks after birth, the testicles are in the scrotum. In about 1 out of 100 births, something goes wrong and one or both testicles don't make it to the scrotum. In these cases, doctors do surgery to move them into their proper location.
During childhood, a boy's testicles can move back up into the abdominal wall, but they descend again. When puberty is reached, the testicles grow and no longer are able to fit into the opening of the abdomen.
During states 1 and 2, the testicles are up near the abdomen, but as the scrotum develops, the cord on which the testicles hang lengthens and the testicles hang loosely below the penis (unless you are cold or engaged in an activity). It is sometime during stage 3 or 4 that there is a noticeable hanging down of the testicles.
The corona (or sulcus) is lower edge of the glans at the end of the penis. At birth the glans is distinct, but not developed. During stage 3, the glans begins to develop a rough texture and during stage for the corona flares out to give the bell-shape glans. The flare is more distinct during erections.