tako,
in my experience with working with both pre-pubescent males and females, i have noticed that they both have what is called 'chest voice' and 'head voice'. in the case of these two groups, as well as adult females, the timbres of these two areas of the voice are more similar than these two timbres in the adult male voice (most would probably agree so far). in general, the boys, the girls and the women sing in both 'head' and 'chest', with the problem of going from one to the other without sounding or feeling as if they are 'switching'. without going into detail, my approach to this problem, in all voices, including adult males, is to have the student begin 'switching' earlier and finish 'switching' later.
the students i find most resistant to this process are men who are uncomfortable singing in 'falsetto' and women who are interested in, for want of a better word, 'rougher' pop music where the aesthetic is more akin to a 'belting' tone than a 'headier' tone. so, i suspect the 'loss of their soprano voices', in men, as you put it, is more likely due to cultural factors than to physical factors, as it affects both men and women (and i suspect that its affect on women may be more pronounced in the past forty years relative to the development of pop music).
mike
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