Two twins, Don and Erica, are $20$ years old. They leave Earth for a planet $15$ light years away. They depart at the same time from Earth, and travel in different spaceships. Don travels at $0\mathrm{.}8$c$,$ while Erica travels at $0\mathrm{.}4$c$.$ What is the difference between their ages when Erica arrives on the new planet?