If my memory is correct, and it probably isn't, the 25% of your body weight originally came from the Swedish? military. After conducting a series of tests they determined that at or below 25% of ones body weight there was not an significant change in ones physical performance. Over 25%, bagn a marked decrease in efficiency and performance. I wonder if there is a link to where I read about this. Even if my memory is correct, this would be for adults in military shape. Extending the conclusion beyond the research data increases the error.

EDIT:
A quick search yielded this: http://eprints.ru.ac.za/669/1/Load-stress.pdf

A fascinating read. It doesn't verify my memory. In fact it somewhat contradicts it. So my memory, as assumed, was wrong. On one page there was a reference to 25kg for an in shape young male. (The research was done in 1977). I wonder if this morphed into 25% by the general hiking public?