Fly at 99% the speed of light for ten years, and you'll return to an Earth that has aged 70. You will outlive everyone you left behind, but the journey itself will shorten your life.
This paradox is driven by Albert Einstein's Theory of Special Relativity. The faster you move through space, the slower you move through time relative to a stationary observer. This phenomenon, known as time dilation, creates the famous "Twin Paradox." However, traveling near the speed of light does not increase your personal, biological lifespan. From your own perspective inside the ship, your watch ticks normally, your cells divide at their standard rate, and you will still age and die after your normal 80 to 90 years. You do not live longer; you simply travel into Earth's future.
While the physics of high speed does not inherently extend your biological clock, the environment you must travel through to achieve those speeds actively deteriorates it. Space is fundamentally hostile to human biology.
The primary threat to lifespan in deep space is ionizing radiation. On Earth, the planet's magnetic field and thick atmosphere protect humans from galactic cosmic rays and solar flares. A spacecraft traveling between stars would lack this planetary shield. Without massive, heavy shielding—which makes accelerating to high speeds incredibly difficult—deep space travelers would be bombarded by high-energy particles. This radiation tears through DNA, causing severe cellular damage, significantly increasing the risk of fatal cancers, and accelerating degenerative diseases like cataracts and heart disease.
Furthermore, if a spacecraft is coasting at high speed, the astronauts inside are in a state of continuous freefall, or microgravity. Prolonged weightlessness causes the human body to atrophy. Without gravity constantly pulling on the skeleton and musculature, bone density drops rapidly, muscles waste away, and the cardiovascular system weakens because the heart no longer has to pump blood against gravity. Even with strict daily exercise regimens, astronauts returning from just a few months in low Earth orbit require extensive rehabilitation.
Ultimately, space travel at relativistic speeds alters when you die relative to the calendar on Earth, but it does not buy you more years of life. Physics shifts your timeline, while the harsh realities of space will likely take years off your natural biological lifespan.