Why? Because of passion!
Passion is the same @ 18 and @60, 70, 80...and so on...
Why should "old fellas" give up their passion? To give the way to the "young fellas"? As I lived this passion during all my aviation career (I am 63, approaching 64) why I should give up? I still enjoy every day flying "my" B744 and have never considered flying as a job...maybe those who are asking this "right of way" have this idea...but for jobs, written and accepted rules (contracts) apply...not moral rules...
With all respect, I don't consider that a valid reason to step down...only the medical and the proficiency will force me to do that...and, of course, if my employer still agrees.
After terminating my "commercial career" I plan, always driven by passion, to get into the "business" aviation world...of course, again, if I'll comply with medical/proficiency...and I won't have bad conscience problems...it's my passion which makes me happy.