Lets assume that they travel at a speed near 20,000 miles per hour. This is about 9 kilometers/sec or something like 0.00003 times the speed of light. This produces a special relativistic time dilation factor of 1+.5*(.00003)^2 = 1.00000000045
If we multiply this by the 6 month trip, then we get that the astronauts would be about .007 seconds younger than if thay had stayed on the Earth. There is also a slight gravitational time dilation effect ( 1 - 2GM/rc^2 )^1/2 because their trip was at a lower gravitational field than on the surface, but this is not very large compared to the special relativistic factor.