male pattern baldness is caused not only by DHT, but also by sensitivity to it
I agree that DHT is there at full levels in our bodies immediately after completing puberty (age 16 on average - variation 13-19). But the hairloss is due to cumulative, not immediate effect of DHT (except in the very rapid, early onset, NW7-by-25 variety of male pattern baldness) Even non-balding guys do not lose their temples immediately after puberty at age 16 or 17, as some sites suggest "temple recession is a part of puberty" but lose them only after accumulated exposure to DHT for 5-10 yrs (most non-balding guys lose some of their temples anytime in their 20s, usually mid-to-late 20s) This is also why hairloss still continues even after "andropause" in men (around late 40s), although at a slower rate, because the sensitivity of the follicles increases somewhat to counteract the effect of less DHT production.
And it is true that a higher percentage of guys with lots of body hair tend to bald than those with moderate or little body hair. But this is not always true - there are exceptions. Take an example of two of my friends, both age 18. One has very little body hair , and does'nt need to shave yet , but in spite of this, he has receded a little at his temples. The other one is a hairy beast, with loads of chest hair, a strong-growing beard(it would not be excessive for him to shave twice a day) and even some back hair. But he has thick curly hair, and has'nt receded at all as of now.
Thus it is the sensitivity of the follicles to DHT, not the level of DHT itself, which is more important.
Cheers,
Arjun