>>281968100
Why do you think that? Bonney reverted the old people in her crew down to their prime age. Bonney was able to change herself to a timeline where she was adult and Nika.

Why wouldn't Garp be able to do it? He's a playful eccentric, basically just old Luffy who can also use Nika form. If anything he'd probably be better with it than Bonney given how strong his body and haki is, and how he's probably even less mature than her.