It may be that Kerry described the soldiers as kids, because from the perspective of his age and given the life experience he has attained in the intervening years that he considers that time as distant as his own childhood. From the perspective of a man his age, those between the years of say 18-22, are kids. YES, at that age you are legally allowed to drink, gamble and fight wars and are adults, but from the eyes of a man that is a generation older it is easy enough to see how he may have chosen that description.
The phrase "college kids" has certainly been tossed around enough in popular culture, and it is more of a way that a person looks back upon their youth at an early stage of life experience, than it is to imply their need of protection. This is not to say that it was a poorly chosen phrase, as it certainly was. It is also true that a soldier in a war gains experiences at a much more accelerated rate than those who have not been subjected to such intense conditions. However, I would not be surprised if many of you soldiers and others of that age will look back at this time of your life and realize how much you had learned and grown in the intervening years. I also would not be surprised to hear any of you, as you progress in years not to look at people in their late teens and early twenties and refer to them as kids.
Granted, the political arena is not the place to throw about words in which all implications have not been carefully considered, especially those that can be construed with less than optimal perceptual value.
Words are the currency of politics, and regardless of the intent or the intended audience, those that are poorly chosen are likely to affect the voting public.