When do boys' testicles drop?
The answer depends on what you mean by "drop." As you were developing in your mother's womb, you testicles formed inside your abdominal cavity. Just before birth (usually during week 28), they drop through holes in the abdomen into your 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. Without this surgery, the testicle will not function after puberty; hence, it will impact a boy's fertility as well as his hormone production. Please be aware that boys with undescended testicles have higher than average risks for testicular cancer. They should be extra diligent in checking their testicles for signs of cancer.
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. Initially 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 activity). The growth process takes about one to two years before there is a noticeable hanging down of the testicles.