“Turnout” is usually defined as the percentage of legally eligible people who actually vote. So defined, turnout was higher back in 1900, when no women and few African Americans were permitted to vote. But if we want to measure how democratic the society is, it’s better to ask how many people voted out of the whole population. By that measure, 2020 will be the best year in US history.
This graph assumes that 149.5 million Americans voted in 2020, although that number may actually be higher. It implies that 45 percent of the people voted. That beats the previous high of 43.9 percent in 2008. Note that the ideal rate would not be 100%, because the population includes people of all ages, even babies. But 45 percent is not high enough.
Population estimates from the decennial Census, with my own linear estimates for the intervening years. Number of votes cast from Dave Leip.