This assumes a bit too much rigor in common language. When most people wonder "what's guaranteed in life", they really mean "what kinds of events and causal links are so common in my context that I should give high probability to their occurence".
The common answer "death and taxes" shows this well. One of them is far more likely than the other.
There is nothing guaranteed in this life. The only thing that is guaranteed in life is –death.
Now one might counter this and say – "That's not true! If I don't eat food, it is guaranteed that I will be hungry." But you could keep feeding yourself continuously after different points of time so that there is no hunger as such. If you stay hungry for very very long, you are guaranteed to die. Even if you don't stay hungry, you are still guaranteed to die.
One could say that growing up of a child or any being is guaranteed, but there are babies that die every year due to some of the other causes. So even growing up in the true sense is not guaranteed. One may say that new cells replace old cells rapidly in the body which is guaranteed to happen, but this is again a variation phenomena of death. This is because when new cells replace old calls, the old cells actually die as we know from the scientific perspectives & studies.
Regardless of what you do, or do not do, death is a certain certain thing. That's why I call that it is guaranteed in life. And it is a separate question whether death arrives earlier or later in life.
Rest of our life processes and events are largely uncertain. But why even bother to ask what is guaranteed in life?
It is because once we know what is guaranteed in life- we must become aware of this fact, accept this very reality and work backwards from it and ask- death is guaranteed to come to myself some day, what I must do before death? What really matters to me and what should be really doing?
Awareness of the fact that death is guaranteed, does not mean that we take this thing negatively and become depressed. It means to become aware of the seemingly negative and painful aspects of life so that we can then deliberately focus on the positives in life. And help transform negative to positive in life such as by helping people also become aware of this reality and being compassionate enough to help reduce suffering.