Ven didn't die, but since his heart was damaged in the battle against Vanitas now he's sleeping while waiting for Sora to set things right. He's now stuck in the Chamber of Waking in Castle Oblivion, which can't be accessed by anyone but Aqua (Org XIII was looking for him in CoM and Days but they failed).
I don't understand the concept of "aging" in KH games. Disregarding Disney characters, Sora, Riku, Lea/Axel, Isa/Saix, Ienzo/Zexion and Kairi get older but Aqua looks the same 10 years after (we can assume that 10 years have passed since she fell into the darkness since she meets Ansem after the events of KH2).
I think Braig and Answem the Wise are much more involved with this thing that they seem since both of them got their eye color changed from black/blue to gold. Maybe Master Xehanort is possessing/controlling them (notice how Terranort's eyes are brown and not gold in the secret ending when he wears sciencist's outfit).
Lastly, anyone else notice that the dialogue between Ven, Terra and Aqua is strikingly similiar to Sora, Riku and Kairi's from KH1?
"Come on Ven/Sora, I thought you were stronger than that.", "Give me a break Kairi/Aqua" and "Sora/Ven, you hopeless sleepyhead".