I guess a tractors true lifespan is when it gets to the point when nobody can be found that is willing to purchase and repair the machine at any cost, not even for free.
As a ball park guess, I would think in terms of engine hours, not years.
For a 35 hp tractor, I would guess by 10 to 12,000 hours, it would be in such poor shape It might not be worth the expense of fixing.
A larger Ag tractor like a JD 93xx would still be out working the fields at 10k hours, probably lots of Big Ag tractors out there with 16,000 hrs still working fine,
Plenty of examples both ways that are exceptions to these numbers, but that is my guess as a place to start.