**Core Concept**
Undescended testis, also known as cryptorchidism, is a condition where one or both testicles fail to descend into the scrotum by a certain age. Orchidopexy, or orchiopexy, is a surgical procedure to bring the undescended testis down into the scrotum. Early intervention is crucial to prevent potential complications and improve fertility outcomes.

**Why the Correct Answer is Right**
The American Academy of Pediatrics recommends orchidopexy for undescended testis by 12-18 months of age. This timing is based on evidence that early surgical intervention reduces the risk of testicular cancer, infertility, and other complications. The procedure is usually performed between 6-12 months of age, but it's essential to balance the benefits of early surgery with the risks of anesthesia and potential complications. The exact timing may vary depending on individual patient factors and the surgeon's expertise.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it's too early for orchidopexy. Most pediatricians and urologists recommend waiting until the child is at least 6 months old before scheduling surgery.
**Option B:** This option is incorrect because it's too late for optimal outcomes. Delaying orchidopexy beyond 18-24 months of age may increase the risk of testicular cancer and other complications.
**Option C:** This option is incorrect because it's not a universally accepted age for orchidopexy. While some surgeons may recommend surgery at this age, the American Academy of Pediatrics recommends between 12-18 months.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that undescended testis is a risk factor for testicular cancer, and early intervention with orchidopexy can significantly reduce this risk. However, it's also crucial to note that not all undescended testes require surgery, and some may resolve spontaneously with time.

**Correct Answer: C. 18 months.**