About Mickey's age... Apart from the Timeless River point, let's not forget that this IS kingdom hearts after all, Sora and the gang are not the only protagonist. The whole story (apart from Sora looking for his missing friends) are about a darker plot in which King Mickey's been pretty much involved in. So, far all we know, King Mickey might have been there since the dawn of time (in KH's time of course).
And I think Kairi's grandma's story is really important... For one, its basically the only solid thing we have apart from other people's speculation... (the soldiers' name don't count since neither square enix or nomura had released a statement saying they're called Ven and Aqua). And for all we know, the battle might have been about THAT battle...
And another thing, one of the member of OrgXIII mentioned something about other keyblade wielders... So it might be possible that they've been there since the beginning too... (Otherwise, they wouldn't have met OTHER keyblade masters, would they?) Unless of course, there are some randoms keyblade masters in between... For all we knoe, ansem, etc. might be freaking ancient... 'Cause people in disney don't die 'cause they're old. Otherwise, Merlin and Yen Sin wouldn't still be alive...