Actually, this time these are reasonable questions!
Why didn't Mickey or Yen Sid tell Sora about [...] Ventus, Terra or Aqua?
That's a good question. The answer is about as basic as Mickey and Yen Sid are pretty bad at keeping everybody on the same page. Also they never really understood the true depth of the consequences of Xehanort's actions until post-KH2...
Mainly it is Mickey's fault for not delivering Naminé's message to Sora properly and instead left it vague. With the letter and everything that went down in KH3D, Sora is at least aware of his role in everything so...he knows enough.
why did they wait until 2.0 to tell Riku and Kairi
0.2.
Because plot. Mainly because the conflict against Xehanort has reached a boiling point, so there is no more room for secrets and half-truths.
Also poor foresight.
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?
Mickey had no clue Riku and Sora had anything to do with Terra, Aqua, or Ven so it likely never occurred to him to tell them. He didn't even know they were in need of saving.
Also, nobody would have known that Sora and Riku would have ended up in the Realm of Darkness at the end of KH2.
It wouldn't have been any help, anyways. Aqua and Ansem the Wise are on the shore opposite of the shore Sora and Riku were on. They wouldn't have been able to find them even if they had tried.
Was they waiting for Sora to get older and stronger and do the Mark of mastery?
Besides poor foresight, maybe? Yen Sid states flat out that Riku and Sora are self-taught and were unaware of the true power of the Keyblade. TAV were trained––one was a master, another at the level of a master, and the trainee was able to keep his own in a battle alongside master-level companion––and they still couldn't defeat Xehanort. What could Sora and Riku do while not even being at that level?
Any earlier than post-KH2 and they wouldn't have even been able to do anything with that knowledge since they were dealing with a lot of things at the time (i.e. the Organization, Heartless, lost friends, a year of life/memories lost, evil villains, etc.)
Overall, it just results in being a matter of poor story planning. Nomura is piecing this all together as he goes.