According to actuarial tables, Biden can expect about another seven years of life. Trump can expect another nine. Looking at the raw actuarial numbers both have a good chance of living to see their successor. Also both men have access to the best medical care.
The clock running faster for Trump than it is for Biden.
Subtract from whatever years Trump has left the impact of Trump's obesity. Trump has been obese for years. His diet sucks. Obesity can take years off a person's life.
Then there is stress. Stress also shortens a person's lifespan. Trump may pretend that he isn't troubled by his arrests, legal woes and the possibility of being incarcerated, but it must be stressful. It must cause a sleepless nights. It Trump isn't stressed out, then his grip on reality is severely in doubt.
Obviously, any president can die while in office. Ironically, it's our younger presidents who have died (or been killed). If you are less than enamored with Kamala Harris, consider the sort of wingnut that Trump would select as his veep.