It is still unknown whether or not amyloid plaques are actually a cause of Alzheimer's but...
fragments clump together to form plaques, and once they reach a certain size they can become too large to be cleared from the brain and they could remain for long periods of time. It has also been shown that focused ultrasound can be used to temporarily open up the blood brain barrier to help clear plaques from the brain [1].
It may be that sleep deprivation is unsafe at any age and might increase risks long into the future, although older adults are probably at a higher risk if the ability to clear is degraded at older age.