What is the average age for a boys testacles to descend?

Zero, sort of...: A normal boy has testes descended before birth at full-term, which means the average NORMAL boy's age is zero when his testes are seen for the first time when he is born. We don't "average in" the ages of other crytorchid boys (those with undescended testicle(s)) to get a number, because the number we get is not useful for anything. That said, a urologist can be consulted at age 6 to 9 months.

During pregnancy: The regular pattern has the testis entering a canal in the groin at 28 wks and entering the scrotum by 32 wks. Some 30% of early premi's and 3% of normal newborns have undescended testis on one or both sides. Most will descend by 1 yr. Finding & relocating them to the scrotum is usually desired by a year of age, especially those who have not even entered the canal by that time.