I believe Van Damme got back into shape for the new Universal Soldier film that was done some months back in Louisiana which lead to Dolph vouching for him on the Expendables. His reputation from his attitude in his glory days cemented his career for many years. He has his act together and has for some time, its just taken the right people to realize it.
In regards to Arnold, if he had another six months to train I think he would be back into T2 shape. As crazy as that sounds, he was slimmed down for T2. For T3, there was some "Hollywood magic" involved. The main reason was more health related at the time than anything else. Many dont comprehend he had a back injury in the late 90's that put a big damper on his physical fitness. As I keep saying muscle memory is a hell of a thing. Combined with a strict diet and supplements, he could get back into T2 shape even being near 65 years old.
On a side note, I dont believe he wants to get back into that shape. He wants to do other films and enjoy life as opposed to be tied down to a weight room again. But of course thats just my opinion, I could be wrong.