I'm pretty sure she will get her spotlight and redeem herself in this arc, she is probably only behind Luffy and Zoro in panel time, thanks to act 1. and even in act 2 she got a pretty important role.
Or do you mean grow up like physically? I think it fits with having straw hats from different generations, and we already have several older generation members, but none one generation younger than Luffy, and it is a cool contrast for when Jinbei finally ends up on the ship, him being the most experienced pirate, and Tama having basically zero experience.
Doflamingo had a 10-year-old (biologically) so why not?
I disagree.
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Luffys own experience is what we should value in crewmembers. He, Ace and Sabo may have been rebellious kids who wanted to become pirates, but they didnt so so until coming of age. This was a huge part of the reason he couldnt join Shanks crew, and the reason the story is set when he is mature and ready to go on on an adventure. I would also like to think Sabos early death would have been a lesson of the risks of pirating at a young age- Luffy understands the value of being able to protect his crew as well as trust his crew to protect themselves and each other. Characters like Tama and Momo are shown to be defenseless the entire time.
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Doflamingo is a bad guy who exploits children because of his own history exploited as a child by Trebol. He has a twisted world view. He is not a good example to follow.
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They are in the new world now they need to add stronger members with experience to upkeep that idea that they are jn a dangerous place. Kids (and sometimes old people) in OP are generally treated as macguffins. They represent the defenseless that are exploited by powerful enemies, the ones that Luffy saves to show he is the hero. Kids on the crew would make the world feel less dangerous since a kid can join the pirate adventure in the new world. Its a mixed message and a hard sell.
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New crewmember should be stronger than any Strawhat ally we met in Dressrosa. There, we had any number of potential crewmates, but as strong or popar as they are, none joined directly. I think this is why Jinbei works as a member, he has rank and experience and can still join in the younger teams fun. Kids like Tama or Momo would add little to the crew unless they physically grow up. Otherwise Nami or Usopp being saved by a kid would forever put them to shame. And if the kid needs saving, they will be unpopular as a crewmate. At least older members have the guise of experience to let them do heroic things even if they are not strong, but a child does not have that yet and we are beyond giving a new character room to grow (physically) in the series. We are building up to Blackbeard
I mean its not like Oda adds a lot of crewmembers. Its been YEARS since the last one joined, no one wants a weak new member after all this time.