The following contains spoilers for One Piece Chapter 1051, "Shogun of Wano, Kozuki Momonosuke" by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English from Viz Media.
As of One Piece Chapter 1051, the hard-fought battle for Wano appears to be over and Kaido's reign of terror has ended. The remaining retainers and Hiyori greeted Momonosuke as he declared himself the true Shogun, standing before his people for the first time since his family's demise. Things won't be easy for him -- while he may appear to be a 28-year-old adult, he still retains the mind of a child. Wano needs a strong leader to repair the damage Kaido and Orochi have caused, but can Momonosuke lead his country to prosperity?
Momonosuke and his father Kozuki Oden's retainers were sent 20 years into the future by Kozuki Toki, Oden's wife, with the use of her Toki-Toki no Mi powers. Momo was only eight when he traveled to the future and was terrified to leave his mother and sister, but he knew it needed to be done to preserve the Kozuki Clan and give Wano a chance at freedom. He posed as Kin'emon's son to hide his identity while looking for someone to help defeat Kaido. Throughout his journey, he has been immature and easily upset over his circumstances.
While Momonosuke is still afraid, watching Luffy and the others be willing to sacrifice themselves for the sake of Wano inspired him. Despite his fears and with Luffy's encouragement, he knew he needed to be strong for his people. Momo convinced Shinobu to use her Juku Juku no Mi to age him up 20 years -- the age he would naturally have been had his mother not sent him forward in the future. The Juku Juku no Mi (Ripe-Ripe Enticement Jutsu) allows the user to age and decay a target at will, but if it is a person, the mind is unaffected and the aging cannot be reversed.
While he overcame the struggles of war and is physically an adult, Momonosuke is still mentally a child. He has been through more than most people -- even by One Piece standards -- but his mind isn't fully developed or matured. Learning to be an adult when still mentally a child is a challenge he will have to overcome if he wants to be the strong leader Wano needs. He won't have the chance to experience the wonders of youth and the lessons that come with growing up; instead, he will have to learn those lessons as an adult.
Despite this, Momonosuke has displayed the potential to grow as a leader and make important decisions. He elected not to open Wano's borders just yet, though the way he worded it implies he doesn't intend to keep the country closed permanently. It is probably for the best, as Wano needs to heal from 20 years of tyranny and pollution. The people need time to relearn how to be free, the leadership needs restructuring, and its factories need to be torn down. The crops need tending to, and the children who lost their families must be taken care of. Luckily for Momonosuke, he isn't alone and has the support of Hiyori, Kin'emon and the others, who will surely help guide him on his new journey as Shogun of Wano.
