Interesting question.

I imagine someone has already thought of this from a tax point of view.

Maybe some % of their net worth divided by their age (working age?) - say if they have 100 million dollars and are 60. 60-18 = 42. We would fine X% of 2,380,952.38? The idea being an attempt to come up with what their yearly salary could have been based on current value.

I am sure there are some excellent flaws with that approach as well.

Fun to ponder.