is is ever mentioned when you're officially an adult in the one piece world? because Luffy sets sail at 17 and Koby joins the marines with 16, so perhaps you're considered an adult by that age? But according to Sabo you become a world noble at 18? (aren't you one either way if you're born into a noble family?) But then again, if Sabo wasn't (as some say) planned from the beginning, perhaps one should not place much value in this reasoning - idk. It rather feels like you're considered an adult at 16/17... since the other younger strawhats were also more or less on their own out their (or in Sanji's case having an actual job)... and then again there's Bonney who is 12 by the time she arrives in Sabaody with her own crew so... then there's that^^ (but she might be an expection since she usually looks like she's an adult)
Hi, thanks a lot for your question!
I don't think there's a set age for when you're an adult and in a lot of cases it probably doesn't really matter. Why? Because we're in a world of pirates and many children and teenagers have to learn how to fend for themselves early on, that's just the cruel reality. I'll try to give some ideas regarding the examples you gave and then name my headcanons for what I think the more "official" age limits might be.
First of all, I'd say Bonney is outright disqualified because of her devil fruit. One Piece doesn't really have a form of ID, so they can't check her age, but even if they could... you can't tell me Bonney wouldn't have a fake ID or get her way either way because she's a pirate.
Luffy setting sail at 17 is most likely because Ace sat sail at 17. Also, Luffy is Luffy, so he probably doesn't care about age limits.
I wouldn't say that Sanji having a job is really an issue here because cabin boys are common amongst pirates and so are kids helping out chefs in kitchens. Also, he begged for that job on his first ship and then got given the job by Zeff when they opened Baratie. They did open Baratie together, even though Sanji was a kid, so I can't see him not getting a job here. Who knows if he actually got paid either. I don't think we can apply our definition of what constitutes a job in this case.
Now with Sabo and Koby is where it gets interesting, so this is where I'm gonna get into my own headcanons.
I think the One Piece world might be working with similar age limits/requirements as we do here in Germany. That means you're legally considered of age when you're 18, but you already get to make some important legal decisions when you're 16, very rarely also as early as 14, usually with a legal guardian's approval.
Why do I think this is the case? Being of age with 18 lines up with what you said about Sabo and becoming a Celestial Dragon. Yes, you're born a Celestial Dragon but you probably only get the full privileges of one when you're of age - so, most likely 18. Koby joining the military 16 wouldn't work here in Germany, but it's only one year off and I'm pretty sure the bar used to be lower - which, One Piece takes place in a more historical-esque setting, so we can probably assume 16 would be the minimum age requirement. Especially with the Marines being this present in day-to-day life and supported by the Government.
Why did I say Germany instead of any other country with the same age limit? Well, on one hand because I live here, but also because of one specific reason: Alcohol. Usopp (at least in the live action) got completely wasted at Baratie and I'm pretty sure Baratie is a place that would check your ID if you wanted a drink (at least if it's running normally, not like when Koby and Helmeppo showed up). Usopp is 16 but he still managed to get wasted, most famously off that huge bowl of fruit punch or whatever that was. I'm very sure that that stuff would fall into the category of drinks you'd be allowed to get at 16 in Germany. Light alcohol like wine and beer at 16, hard stuff at 18. Some more hints towards it being like that in the One Piece world too are that Zoro and Nami had no problems getting rum, which is hard alcohol, and they're 19 and 18 respectively. Helmeppo was also easily able to order shots for himself and Koby and he's 20 at that time. What we can definitely be sure of is that the limit isn't 21.
Whatever the age requirements are, I believe that, while they would be universal for anything involving the Marines or the Government and probably be enforced too, in the realm of pirates they don't really matter. Like, as a bartender, are you really going to deny a pirate crew some vodka for their 14-year-old cabin boy? It's either you give alcohol to a kid and maybe pay a fine or you lose a limb - I think the answer is very clear. Different kingdoms could probably adjust the age requirements to their liking, but I don't think they could lower them too much. But I don't know, that's just speculation at this point.
All in all, being in adult in the world of One Piece probably depends a lot more on the respect people have for you and how independent you are than your age.
I hope these rambles helped at all - have a lovely day!
