Gaimon is gonna be maybe not a nakama, but he's gonna be in the strawhat fleet.
I need to see Gaimon's chest-based pirate ship.
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Shanks didn't let Luffy join his crew because it was simply too dangerous for him. And while he didn't understand the why back then, now he understands after losing Ace and almost all of his Nakama. So you expect Luffy to ignore the lesson he learned from his idol and all of his own experiences and let a defenseless child like Otama join while riding through the most dangerous sea in the world? C'mon guys...
Shanks didn't let Luffy join his crew because Luffy is a crazy person and would have gotten himself killed by challenging enemies he couldn't beat.
Momo just became an adult, basically, and managed to blow up an Admiral's Logia form.
Shanks, Buggy, Momo, and Tama have proven that Oda doesn't mind having children characters sail with pirate crews.
Bonney and Shinobu can deal with the age component with Tama.
If Tama has learned Ninjutsu the next time we see here, she's probably joining the crew, especially due to her connection to straw hats and Ace.