Two 20-year-old twins are hanging out on Earth, and one decides to go on a trip and a spaceship. The spaceship travels at 99% the speed of light for 1 year according to the clock (clock 1) on the spaceship. The twin on earth compares her clock (clock 2) to the clock of her twin (clock 1) and notices that her twins clock indicates seven years have gone by. If we reverse the situation, in other words, if the twin on earth measured one year on her clock (clock 2), the age of the twin on earth would be 21 years old and the age of the twin on the spaceship would be _____ years old.