I don't think they could get enough older people (I mean Renny-Jerry-Kent old enough) who are physically fit enough for the challenges.
I don't need a 'theme' to the casting. And I don't mind if the people are attractive, I mean we're going to be looking at them around the clock for 2 1/2 months. But how about simply finding and casting a wide range of ages and types, like they were able to do for BB10? That to me was a near-perfect cast in terms of variety. Since then it's been mostly a younger bikini-bodies club.