I think Bonney's DF has to do with transfer or take or add, like: She's able to take years of age from a person(turning that person into a kid, depending how many years she took from that person), then she can transfer it to another person(which will make that person old, depending on how many years she adds to that person age, and this will make the person get older or become an old man). This will explain why we saw both equal numbers of Kids and Old men.
! http://images1.wikia.nocookie.net/__cb20130209040156/onepiece/images/4/42/Bonneypower.png If you count the number of oldies there and kids there you see they are equal.
Edit: well it's more clearly shown in manga: http://www.mangapanda.com/103-2614-17/one-piece/chapter-507.html
That is an interesting theory but when she saved Zoro it didn't seem like she 'took' any years from anyone there nor did she 'give' the years back when she transformed back into an adult. Though im guessing you're going to argue that happened off-screen? I think you're reading too much into the whole equal numbers thing, I saw it more as a showcasing of her ability to change people's age either way (old/young).
Cool theory though.