Travel at 99% of light speed and time runs seven times slower. Come back and your twin has aged more.
Special relativity says the faster you move, the slower your clock runs. At 87% of light speed, half rate. At 99%, one seventh. At 99.99%, one seventieth.
GPS satellites orbit at 3.9 km/s, losing 7 microseconds a day. Without correction your position would drift 10 km within a single day.
Each twin sees the other’s clock as slow. It looks symmetric, yet only one ends up older — because only one underwent acceleration when turning around.
In 1971 atomic clocks were flown around the world aboard airliners. Prediction and observation agreed within the margin of error.