Why did Muffy wait so long?

[WindAnnWise]

He waited until Simba was grown and mature enough to handle the truth. Telling him the truth as a cub may have just freaked him out more and gotten him more confused.

[cleargreenwater]

I confess to not reading the whole thread, but I don't think Mufasa *could have* materialized to Simba without Rafiki's intervention, and if Rafiki did not know Simba was alive & thought him dead until getting his scent/omen on the seedfloss, then there you go. He didn't appear to Simba earlier because Rafiki didn't know Simba was still alive to be invoking/calling up dead people for him.

[Konga]

Yeah! This is a really interesting topic, Amanda. I've always thought the same thing myself.
Like you guys all said, I believe that the appearance of Mufasa's spirit was both symbolic and realistic at the same time. Perhaps, like you guys mentioned, Simba needed time to mature and to learn these life lessons on his own. For most of his life, Simba had dismissed his past while adopting the "Hakuna Matata" lifestyle. After his argument with Nala, though, he was finally addressing these issues out in the open; it was somewhat of a release for him. I think that if Mufasa appeared earlier in the film, Simba wouldn't have a chance to come completely to terms with his problems.
Haha, I always wondered that as well. It'd be funny if right before Mufasa's cloud disappeared, he was was all like, "Oh, and by the wai, Scar did it!"