I honestly think Time doesn't flow in a Void. Voidsent know *of* Time, sure, but I get the feeling they don't experience it at "home" anymore.

Could explain why Diabolos was surprised that 1500 years robbed him of some strength; perhaps a similar slumber in the Void is just like a pause, keep everything how it was before it started. Would also explain why Unukalhai is still young; that's how he was when the Thirteenth fell, but the Echo or Blessing kept him from turning into a Voidsent.