I've thought of that but I think we always have some sort of stress of one thing or another throughout most of our lives. Keep in mind mostly women suffer from RSD many times the rate of men and men suffer stress in different ways, maybe differently than women and die from stress more often.
To me could it be the nervous system's maturity or peak years after its growth and before it's slowdown or aging? Also, is ethnicity a part of it? How many asians, Africans suffer RSD compared to whites? For example if whites are the dominate ones who suffer most then which european region is more likely to suffer from RSD. English-Irish, German-French, Greek-Italian-Spanish etc... Is there a race or culture of humans who have no record of RSD?