The correct answer to the OP's question is, of course - "it depends".
Physical (and mental, too!) situations are different for every humanoid for a myriad of reasons, after all.
I'm "only" 61 but y'all know the train wreck of afflictions beset upon me in the last decade, so I'm no
one to speak of "how old is too old" for sure.
It's sort of like with cars themselves - oftentimes, condition is a result of
mileage, not
model year.
I've often written here about the journey to get Fred the GTX to some manner of "finished" status, all
the while racing the clock, literally - and yes, that overwhelming drive
not to leave a mess when I go,
having experienced that with my own dad's passing.
And...I made it, thank God.
Further, my wife and I have had so many discussions about my wishes for after I'm toast that there's no
need for them anymore - things just
are, now - and I've had to learn how to wrap things up and just piddle
with this or that small project, not wanting to begin anything I won't be here to see through.
That all sounds negative or even depressing, I realize - but trust me, it isn't.
It's sort of liberating actually - knowing I got done what needed to be done and there's no pressure, internal
or otherwise, to keep grinding away at it anymore.
Would I love to start another carproject, though?
Hell yes, given the financial ability, you
bet I would - but it's ok that I don't.
It's the responsible thing that I don't...it sort of would undo a lot of my recent past efforts, really.
I can live vicariously through you guys and the wonderful work you do to your own cars in order to somewhat
stay engaged, all the same.
