You know it's this kind of stuff that actually makes the tax code so complicated, but because you insist:
An amount of wealth that would provide you a top %0.1 lifestyle if you were to retire today based on a standard 60/40 portfolio (to be adjusted every N years to reflect new data) and actuary tables.
It doesn't have to be this exact policy, but this isn't hard to think something like this up.
We can't vote/sex/drink/gamble because it is assumed that young people sucks. Maybe looking at the statistics government decided 18 years is good? I think if 12 year old can pass driving test then they should be allowed to drive. If 30 years can't pass, they must not be allowed to drive.
So, how about we find objective paramters to determine what should be the best border to determine the elites? May be it is 100x minimum_salary? There are plenty of option I guess.