I forgot to mention that others use legitimately use 25 or 30% in the calculation, but they're usually talking about adults. For kids, I always selected 20% as a rule of thumb, since their bones and muscles aren't as fully developed as an adult's and since, as others point out, kids' size and growth patterns can put them all over the place physically. A higher percentage might well be realistic for taller, sturdier kids.

Does the higher percentage help you replace helium balloons with gear? (I can joke about it, because I was in the helium-balloon brigade, too, not a whole lot of years ago; it was what first attracted me to the whole lightweight gear thing. Before that, I adjusted trip mileage, number of days out, terrain, and hiking pace to compensate - when I couldn't figure out how to get negative mileage, I was left with losing weight. Bummer.)

Like someone else said, as an adult, you can also simply decide you're going to do it, and go ahead. I did, and it worked - it was harder, and less comfortable, but I still enjoyed it. The weight loss was partly triggered by the realization, around age 50, that I had to carry lighter gear or lose weight if I wanted to continue to backpack for another 20 years; a Type II diabetes diagnosis at age 60 was the final push to get really serious about weight loss.