This is strictly "old-school" vs. "new-school" mentality.
Back in the territory days, guys would wrestle into their 40's and early 50's on a regular basis, which is why "old-school" guys like Flair, Taker, Sting, etc., are wrestling/wrestled into their 50's. Remarkably, while the guys who went to that age and had terrible lives afterwards are what you hear about, a huge number of those guys stepped back from their in-ring work (many of them never seemed to completely retire, but would still work the occasional show every now-and-then) and then went on to have long and relatively healthy lives.
The "new-school mentality" says that people can't have careers in active sports past the age of 40 (the whole "40 is old" though process, which is uncomfortable for those of us getting there). While it is true that in the late-30's, early-40's people start to lose advantages in strength and speed, their experience often gives them other edges.
Let me give an example: Samoa Joe is a veteran wrestler, who is 36 years of age and is currently being touted as "part of the future" in NXT. Most people are of the opinion that the day will come (and sooner rather than later) that Joe will make his main roster debut. He'll probably be 36 or 37 when this happens. By the "new-school" mentality, his WWE career will last 2 or 3 years. In reality, Joe will probably wrestle for another decade (and, likely, at a high level) and may one day have a shot at being a main event guy in WWE in his 40's.
All of this is to say that these guys can retire whenever they're ready. I think it's probably time for guys of an extreme (or more extreme) age like Taker and Sting to hang it up, but, if they're still able to go (and this seems to be the case, by the way...both of these men are still drawing crowds and putting on quality matches, haters be damned), then so be it.
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