There are Leukaemia's that hit you only when over 40
Some of those are pretty rare.
So the premise that us oldies would survive is IMHO wrong.
I got one of these (HCL) aged 54. This one is very slow growing (5-7 years but undetectable until year 4) and it totally fucked up my body for almost a year after I finished chemo.
Good luck going to Mars if you have it growing inside your body.