Yes time is slowed down for them...
It would allow you to travel 50 light years and age only a small fraction of that...

Maybe I misunderstood what you are saying , but it is not like that
"Time slowing down " is symmetric
Suppose you are in the year 3000
A 20 years old astronaut leaves the earth travelling at the speed of the light
He reaches a planet 50 light\years far away from the earth and he immediatly comes back
When he arrives at home , he is 120 years old, he did not age just a fraction of time !
Nothing has changed from the point of view of the astronaut (because of time symmetry)
The point is that the time on the earth on his arrival is not 3120 but let's say
3850