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1. Because during normal intrauterine fetal development, they "descend," and migrate into scrotum. If they don't, bad things happen...
2. "masculinization" through Tanner stages I to V (see below Wilkpedia quote) Final answer: **Tanner V** (facial, axillary & chest hair, pubic hair) and bone remodeling.
3. Urethra travels from bladder, past prostate, through shaft to glans. It is the transit route for urine from kidneys/bladder and for sperm/semen from testes/prostate
Tanner I
prepubertal (testicular volume less than 1.5 ml; small penis of 3 cm or less) no pubic hair at all [typically age 10 and under]
Tanner II
testicular volume between 1.6 and 6 ml; skin on scrotum thins, reddens and enlarges; penis length unchanged.
Small amount of long, downy hair with slight pigmentation at the base of the penis and scrotum (males) [10–11]
Tanner III
testicular volume between 6 and 12 ml; scrotum enlarges further; penis begins to lengthen to about 6 cm
Hair becomes more coarse and curly, and begins to extend laterally [12–14]
Tanner IV
testicular volume between 12 and 20 ml; scrotum enlarges further and darkens; penis increases in length to 10 cm and circumference
Adult-like hair quality, extending across pubis but sparing medial thighs [13–15]
Tanner V
hair extends to medial surface of the thighs [16+]
testicular volume greater than 20 ml; adult scrotum and penis of 15 cm in length
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