The point of setting a maximum age limit is to restore the approximate relative influence of young people that they enjoyed during the very successful years of US democracy during the 19th and early 20th century. The upper age limit must be low enough to exclude sufficient number of people from voting so that the younger voters can have an appropriate level of influence on voting and policy-making. According my calculations from census bureau data, about 32% of the population is over 55, but only 16% is over 65, and only 7% is over 75. The 55 year age limit results in a significant shift in voting power to young people. Setting the age at 65 is much less effective and by 75 there is hardly an affect at all. In terms of numbers, 55 works great as an upper voting age limit.
Assuming that a democracy is strongest when the demographics of the eligible voters are younger, we can redefine the eligibility for voting rights from the current eligibility to all adults to a new eligibility of all young adults. In other words, we need disenfranchise adults after they reach a certain age. This mimics what nature did for us in the 19th century. Older adults will continue to enjoy long lifespans and pension-like benefits. They will lose the opportunity to vote after a certain age.