You guys are overestimating jewelry Bonney DF, I mean there is noway her DF ability can be permanent, if she loses consciousness or Dies everyone she used her ability on will go back to normal(just like in FI, that fish lost consciousness then Noah started falling, same with Shiki), so it's stupid for People to want Bonney to make them young again, if her ability wears off those old people will start going back to there age but it will be too late cuz they are way over there age, so they all turn to ash and disppear from this world. (You can't put your faith on some pirate who may lose consciousness or die anytime). And I believe this why Z never asked Ain to make him younger cuz if she did then when Zoro knocked her out, Z would've gone back to being an old man and guess what? It turns out that Z is not used to fighting at old age n he gets owned.. Jewelry Bonney's DF is only good to herself and I think she's important cuz she knows a lot, not cuz she can make geezers younger.
who knows? maybe her fruit effects really last permanently.