Both clocks tick together.
Both twins start the same age.
GPS satellites
Their speed slows their clocks by about 7 millionths of a second a day. Engineers correct for it, or maps would drift roughly 10 km a day.
v ≈ 3.874 km/s, r ≈ 26,562 km · speed -7.2 µs/day · height +45.7 µs/day (a later lesson) · net +38.5 µs/day · c × net ≈ 11.5 km/day · clock offset −4.4647×10⁻¹⁰ (Ashby 2003)
Muons from space
These tiny particles should fall apart high in the sky, yet many reach the ground. In 1963 their lifetime was measured stretched about 8.8 times.
τ = 2.197 µs · Frisch & Smith, Am. J. Phys. 31, 342 (1963): muons ≈0.995c, 563/h at 1917 m, about 410/h at sea level (sources give 408 and 412); lifetime stretch 8.8 ± 0.8 · model at 0.995c: γ 10.0, mean path 655 m → 6.6 km
Clocks on airliners
In 1971 atomic clocks flown round the world came home out of step with clocks left behind, by the amounts predicted.
Hafele & Keating, Science 177, 168–170 (1972): east -59 ± 10 ns (predicted -40 ± 23); west +273 ± 7 ns (predicted +275 ± 21); speed and height both count
Clocks in a lab
In 2010 a clock moving slower than a cyclist (under 10 metres a second) ticked measurably slower than a still one.
Chou, Hume, Rosenband & Wineland, Science 329, 1630–1633 (2010): Al⁺ optical clocks, relative speeds < 10 m/s (model: 5.6e-16 at 10 m/s); also a 33 cm lift (model gh/c² = 3.6e-17), a later lesson
Calculated, not measured: after six months on the space station, an astronaut ends up about 4½ thousandths of a second younger than people on the ground.
ISS v ≈ 7.66 km/s, h ≈ 420 km · 182.6 days: speed -5.15 ms, height +0.68 ms, net -4.47 ms (model, not a measurement)