@Solid:
Then we have the part of her that makes most people thinking she will not join, her age…
But we have seen some kids in pirate crews, and having a kid in the main party is quite common in jrpgs which Oda also draws some inspiration from, like Eiko from ff9.
For me, Tama's age is not the problem, in and of itself.
Barring what I should hope are obvious exceptions, if the story does a good job at justifying why a child character should be involved in a situation that would normally be left to older characters, I can generally overlook it (again, obvious exceptions). A prime example that I am currently enjoying is Youjo Senki (a.k.a. The Saga of Tanya the Evil), which goes to great lengths to explain why and how the protagonist, despite only being in the 12-13 range by the end of the anime, was able to not only join the military at such a young age but fly up the ranks to a commanding officer position so quickly, as well as how freakishly unnerving she is to her much older peers for having done so. Naturally, the world of One Piece is wildly different than that of Youjo Senki, so I would argue the threshold for justification is lower, but that doesn't mean it isn't needed.
The issue with O-tama, to my mind, boils down to her skills and experience. Unless she's been hiding something, Tama's skill set, besides some typical household tasks, can currently be described as hat-weaving and beast-taming. That's it. While she aspires to be a kunoichi, she hasn't received any training toward that end yet, which means that if she were to join now, the most we could expect out of her would be O-tama the Pokemon Trainer. Everything noteworthy that she can currently do under her own power is dependent on finding a suitable animal (or SMILE-user), getting them under her control, and letting them do the fighting for her. Even now, in the middle of doing the most autonomous thing she's done since being introduced, she's relying entirely on the strength of "O-lin", with a healthy dose of O-kiku and Chopperemon, to accomplish her goal of freeing Luffy.
A bit of ninja-training - enough to get her up to the "Genin" threshold, if going by Naruto standards - would make O-tama capable of fighting alongside her animal companions and give her an avenue of growth independent of them, but there's no realistic way that she will get that level of training by the end of Wano. She now has contacts with people that might be willing to teach her (or find her a suitable instructor), but they're on the verge of a civil war that demands their full attention at present - and even without Orochi and Kaidou looming on the horizon, taking an untrained girl and turning her into even a low-level kunoichi isn't something that can be done in a week or two - it'll be a few years, minimum.
That leads me to believe that Tama is either going to be a "Next Gen" pirate inspired by Luffy, like Luffy himself was by Shanks, or she will join up with the Strawhats a few in-universe years down the road, either as part of an epilogue or after a late series timeskip leading toward the series finale.
I like O-tama a lot, and I think she has a ton of potential… I just don't see her as "ready" quite yet.
@Shiebs:
I'm pretty sure someone said another 2.9 df was a meat meat fruit, which seems to be what tama has… though I could definitely be wrong on that
We haven't been told the name of O-tama's Devil Fruit yet, and setting aside the fact that she pulls them from her cheeks, the beast-taming balls she creates are stated to be kibi-dango, or millet dumplings, not meat.
The kibi-dango => beast taming thing is a blatant reference to the story of Momotarou, so rather than a Niku Niku "Meat" fruit that happens to tame beasts, I'd say it's more of a "beast-taming fruit" that utilizes something she can create from her own body as a catalyst.