Not so sure that age is the whole problem - with pressurized aircraft, cycles do tend to wear out lots of"stationary" bits and pieces.
As for a 600 hr TT pilot flying single pilot? Good grief. That's less than 6 months of actual flying experience (600 minus the 250 hours of CPL training). Not even one cycle of the climate. With so many unemployed, and experienced GA pilot, why oh why would they hire a 600 hour ace.? Daddy made the call can be the only answer.
And yes, 4000 hours is a good minimum number for single pilot operations in a complex turboprop because you cannot always guarantee the near perfect benign weather of W.A. in winter.