And this is the reason you just gotta love chuman.
So, err. The only real plotlike reason for this is that they "didn't thought it was this necessary"?
To bring this really back on track and trying to take a different approach to designating it as merely an "oversight" or "plothole" I'll repeat the OP for a reminder of the questions:
Ventus, Terra or Aqua? why did they wait until 2.0 to tell Riku and Kairi, plus if Mickey knew that Sora was chosen to save them, why didn’t he tell them sooner so when Riku and Sora was in the ROD they could had knew about Aqua and try to look for her? instead of leaving it for another 2 or 3 years. Was they waiting for Sora to get older and stronger and do the Mark of mastery? ANOTHER DUMB QUESTION.
As for the first question, why didn't tell Mickey and Yen Sid Sora, Riku and Kairi about TAV?
Uhm well, let me ask a different question: Why the fuck would they? Before Re: Coded, Sora, Riku and Kairi are to those two only a bunch of kids they work together with on
current dangers and threats.
They are already searching for answers about those three for ten years at the time of KH 1, but more pressing matters reveal itself with Ansem SoD and the Organisation (during KH 1, CoM and 2).
The issue with TAV is
personal for Mickey, as the credits of BBS and his reaction to Master Xehanort callously brushing off their fates in DDD show. To Mickey, Sora & co. were not connected to TAV and thus it was not needed to inform them of anyting.
Always keeping in mind here that we're talking about the time period
before Re: Coded.
The second question "if Mickey knew that Sora was chosen to save them, why didn’t he tell them sooner so when Riku and Sora was in the ROD they could had knew about Aqua and try to look for her? instead of leaving it for another 2 or 3 years." is where the crux lies as it includes an "if".
That "if" must be answered with
no because before Re: Coded, Mickey
didn't know that Sora is destinied to save those three, he did not even know that there was a connection between Sora and TAV at all.
Re: Coded's ending even
shows us this plainly:
Re: Coded said:
Naminé: ...and the ones you're about to see.
(Two boys and a girl appear)
Mickey: It can't be!
Naminé: Mickey, maybe you know how much they're hurting. These three were keys, too. They were connected to the truth behind the Keyblade. And, they're still connected, Sora, somewhere way down deep inside of you.
Sora: Funny, I feel as though...I've met them.
Naminé: Yes, two of them, you have met. As for the third...I never realized it, but...you and he share a very special connection.
Mickey is surprised/shocked to see TAV in the memories Naminé found inside Sora,
this is the very moment he realizes the connections and Sora's importance, not at any point earlier.
The last sentence Naminé says refers to Ven, and that's where Mickey puts two and two together as we see in the secret ending later:
Signs of what's next said:
Mickey: Yen Sid... I think we're finally close to figuring out where Ven's heart is.
Yen Sid: Is that so? Then that leaves only Terra.
In the memories shown by Naminé, Mickey gets to see how Roxas looks like for the very first time (remember Mickey never met Roxas during the year where he was running free), but he knows that Rox is Sora's Nobody and this together with Naminé speaking about a "very special connection" Sora shares with "him" (note the same theme of not saying Ven's name that was already in Days) makes Mickey finally realize where Ven's heart could possibly be hidden.
So this meme of Mickey about "Lol I forgot about TAV" is not expedient. He didn't forget, he just didn't see any necessity in informing Sora & co. about something that he didn't consider their business until he learned in Re: Coded that it IS their business (or at least Sora's business) after all.
So the earliest possible way for Mickey to inform Sora & co about TAV would have been between Re: Coded and DDD and
here is were any "asshole yoda mouse"-accusations can gain a slight vibe of credibility. His letter to SRK was really unnecessarily mysterious and flowery, but that could have been excused if he had briefed Sora and Riku in detail about what he had learned in Re: Coded
before they started with the exam as then Xehanort's plan regarding Sora would have been at least more difficult to achieve because Sora would have been forewarned.
Naminé in Re: Coded warned about
exactly what Xehanort and his nortys did to Sora in DDD so in hindsight it's an error on Mickey's part to not inform them in detail beforehand:
Sora: What? Isn't it weird for somebody else's memories to be inside me?
Naminé: Right, it's not usually possible. When I first found them while repairing your memory, I thought I'd made a mistake. But all the evidence I found proved they belong in your heart, where they've been sleeping a long...long time. One day, Sora will have to call them to the surface. They're important memories...but very dangerous ones.
Mickey: Dangerous how?
Naminé: These memories are too painful. Handled the wrong way, they could damage Sora's heart--even break it. I needed you to find a way to face that kind of hurt. That's why I left the message.
Granted though, I doubt that Mickey was expecting Xehanort to interfere with the exam so not informing them before might actually count as a near-tragical mistake on the mouse's part.
That should answer the OP's question in detail (and hooray, you can do it without any interviews as only some attention to Re: Coded is needed to understand it).