I feel like the Realm of Sleep would explain Ventus not aging, but Marluxia's presence in the Tangled world brought the possibility that Lauriam's survived this long through a completely different method.
Think about it. His elemental abilities are centered around flowers. He appears to Sora in a world based on Tangled, where Rapunzel's hair can restore someone's youth, and the ability originated with a flower. See where I'm going with this? I think that Marluxia is going to be working with Gothel, but not in a way where he replaces the Stabbington brothers. Depending on how long ago the Keyblade War was, Marluxia may have been doing the whole "sing to the flower to stay young" thing since before Gothel was born, and may even have been the one to teach her the song himself.
As for why Marluxia would take the slow path instead of spending centuries in the realm of sleep like Ventus might have been doing before Xehanort found him, I just had another about that. Here's what might have happened: something happens that forces the Dandelions to go into the Realm of Sleep. But they need someone outside, awake in order to pull them out again when the time is right, and Lauriam was given that job.
Man, I wish I seen this earlier but this is a genius theory. This is a smart means of better integrating the Disney properties in Kingdom Hearts with the original lore of the series.
Though I do have one question regarding this, how does Lauriam get involved with Organization XIII throughout all this (assuming that Marluxia is indeed Lauriam's Nobody)?