In a case of pediatric cataract surgery where the lens was not replaced, there is no lens to serve as a barrier between the aqueous and the vitreous. I once asked an ophthalmologist what keeps them from mixing together in that case -- are they of different viscosities, like oil and water, for instance? She said it's something like that, but that with aging the vitreous gradually thins (causing floaters, among things) and they end up blending together. She said this isn't really a problem in practice, as each humour's main function is to deliver nutrients to the right parts of the eye and they can still do that.
I understand that the aqueous drains through a duct, so that part isn't a closed system. But the vitreous usually is a closed system, as I understand it, which is why floaters don't go away. A remedy for floaters that is sometimes brought up is vitrectomy, but that carries risk of retinal detachment (according to Wikipedia, and I think I remember hearing that from an ophthalmologist too).
Putting all of these together, it seems possible that with aging, as the vitreous becomes less distinct from the aqueous, some of the vitreous might end up draining through the same duct that the aqueous drains through, in an eye with no lens to serve as a barrier. Is that the case? And if it is, does it carry with it a higher risk of the things that can go wrong with vitrectomy (especially retinal detachment)? Or does the risk that comes with that procedure not arise with a slow, gradual aging process?
Update: I've seen a reference to the hyaloid membrane, which, in addition to the lens, sits between the vitreous and aqueous. I don't know if this membrane is also removed when a lens is removed or if it remains.