The adult sizes have a 24 inch drop.
That's not actually true. They have a 24 inch total length. That's not the same as a 24" drop (or am I wrong?). In any case, I just ordered an adult SWK for Red_Raven_Studios' 13 year-old son. Of course he's 13 and taller than I am, with a 28 inch waist. I ordered a 30 waist, which he will grow into, and the 24 inch length should be fine on him.