getting old sucks but you have to learn to accept what you will one day become physically should you live long enough. I really think the winner in the age department out of all the actions heroes ever, will be Clint Eastwood. The dude is 81 years old and still leads and active working life. Making more movies as a director than men half his age and really good ones to boot.
As far as Arnold and Stallone, well they both have lived some hard lives physically. Arnold in his private life has broken many bones wrecked his bikes and had open heart surgery not to mention the wear and tear of 30 years of gear(see heart surgery). Arnold was never about the fitness healthy lifestyle that he bullshitted about. It was a means to an end for him. Leave that area for the late great Jack Lalanne.
Stallone has had more damage done to him via the movies than any i can think of. He hired Franco Columbo to train him and soon after tore his pec while trying to out bench franco, got balls for trying anyway. He was punched so hard by Dolph Lungren on set that it bruised his heart. He got his neck broken by Stone Cold Steve Austin on the set of the "Expendables". Both these guys have seen and had more hospital visits than many of us will probably have. I do think that Stallone has made more dedicated to fitness overall than Arnold as it pertains to himself.