Death takes different forms, and that affects how or whether we think about it:
i) Sudden expected death
Above a certain age, you can drop dead from a stroke or heart attack. In that respect, the elderly expect to die, but they don't have a date certain. For all they know, they might have another 30 years ahead of them.
ii) Sudden unexpected death
In the age of modern medical science, there's a presumption against dying young. That's the opposite of the past, when there was high mortality.
As a result, it's shocking when young people die. That can happen suddenly, without warning, in a traffic accident, or due to something like a pulmonary embolism or undiagnosed congenital heart defect.
Because the prospect of death is a safe abstraction for so many young people, that leaves them unprepared in case they're unlucky. It fosters a presumptuous attitude. And it's too late to learn from their experience. They don't get a second chance.
This is the stuff of old-fashioned evangelistic sermons ("If I died tonight..."), and while it's easily parodied, that's a neglected truth. Our forebears, without the benefit of modern medicine, were far more alert to the precariousness of life.
It's sobering to realize that death may be imminent when you least expect it. You thought you had decades ahead of you.
iii) Countdown to death
Then there are cases like terminal cancer where people have advance knowledge about their lifespan. Not a date certain, but an estimate. Within a given timeframe. The clock is ticking–louder.
Moreover, the progression of the illness accelerates the process, which clarifies whether their case falls within the outside or inside range of the estimate. They can see death approaching as the end comes closer.
The prognosis could be off. It might underestimate or overestimate the remaining time. There might be spontaneous remission, or miraculous healing. Still, that's unlikely. The presumption is death sooner rather than later.
In one sense this is worse than sudden death because it can foster foreboding and dread–unlike those who died unexpectedly. In another sense this is better if they take advantage of their prognosis to prepare themselves intellectually, emotionally, and especially spiritually for their impending demise. Unfortunately, many people fritter away their opportunities. They cling to this life, even when that's futile, rather than using their remaining time to prepare for eternity.