"I'm going by voter turnout based on percentage of voter age population https://en.m.wikipedia.org/wiki/Voter_turnout_in_the_United_States_presidential_elections which peaked in 1960 for the period from the Great Depression to the present."
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From your own citation: "Michael McDonald and Samuel Popkin argued, that at least in the United States, voter turnout since 1972 has not actually declined when calculated for those eligible to vote, what they term the voting-eligible population.[6] In 1972, noncitizens and ineligible felons (depending on state law) constituted about 2% of the voting-age population. By 2004, ineligible voters constituted nearly 10%."
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Counting turnout against voting age population rather than against eligable (or better, registered) voters is nonsense.