Note that replacement age is less than or equal to wearout age and that normal wearout exhibits an increasing failure rate for the unit's components.
As long as it is likely to fail gracefully [for a few bucks you might be able to get monitoring devices which warn you of problems] and not catastrophically, I guess you could keep it.
From another angle, if a new unit is $10k and a catastrophic failure of your existing boiler would cause $100K damage, you should get a new unit if the likelihood of a catastrophic failure is more than 10/100 = 10%. The $10k would then be considered an insurance premium against a catastrophe. Of course, existing homeowner's insurance will change this equation.