I have been thinking upon that age range through the various era of our most recent past. If you think about it this is the first era or period of time that 18-25 doesn't really mean young adult
at all. let me explain. what is the average age a child leaves its parents home today? What ways it about 50-60-70 years ago? I don't mean college but to leave and start thier own life with family, career etc? In factI beleive we have more young adults at home still trying to figure out how to live a life then ever before.
Kids were expected to grow up much earlier back in the days then lately, so i would question the adult part if you can define adult as being on ones own.
also the other side of the coin is that the need to define and set guidelines is present in our social structure. even though the question concerning race is suppose to be a non question it is still asked, as are many others that should be better left out of day to day life. Still the corporate and political structure is such that to define is a needed, if not unwelcome necessary evil.
I guess with the blending of past and present the term young adult had to stick some age range. with the new terms 50 is the new 40's and 40 being the new 30's or 60 being the new 50's I guess all the catagories of age is blending into one big mush of number ideas and old time mombo-jumbo!