It is known from Lost that the MIB ...
... can change his appearance into someone who has deceased, been killed.
In one episode then, Shannon gets a vision or visitation from ...
... Walt, who has left the Island per boat to onshore.
I explained this myself - even after knowing of MiB's power - that Shannon is a mentally unstable person and this could also have been through a little bit of crazyness. A vision or illusion.
But shortly after, Sayid experiences the same thing. Even John Locke gets visited.
This means to me that it's not explained for Shannon's instable mind, because it also affects other people, which are way more stable.
AFAIK Walt hasn't deceased, so in my understanding the Man in Black can't take Walt's appearance.
Did I miss something in the story? Has this been explained?
(I've watched Lost know a couple of times, so spoil me! This has always been a question I couldn't answer myself.)