I put my fair share of time googling different word combinations but can't find an answer, help my curiosity Reddit please. 21 seems as arbitrary as 20 or 19 or 22. What were the talking points for the age 21, were there other ages considered? This could be for the time period before the 14th amendment (how did each state determine it) or for it's creation (Section 2 says twenty-one).
I'll try to keep this narrowly focused because the conditions for being allowed to vote, and why they exist, is a broad topic.
By the time the constitution of the US was passed in 1787, there wasn't any sort of agreement on a national voting age, with the states having in general the ability to determine who had voting rights. In a general sense, this meant that practically you had to be a white male landowner over the age of 21 to vote (Ratcliffe).
The voting age of 21 actually pre-dates the United States and can be found in United Kingdom documents & laws as far back as the 17th century (CB Macpherson). The United States essentially inherited this as an age requirement by tradition, as did a number of other colonies like Canada, Australia, South Africa, India, and more (A Blais et al). The reason they generally framed voting requirements in traditional British terms, according to Ratcliffe, was the argument that sufficiently landed people were those with vested interests in the community's welfare. People like laborers, unskilled workers, slaves, etc were not considered to have a real 'stake in society' or a permanent interest in the community (Ratcliffe).
With the revolution, things changed as they needed to ensure popular support, so things opened up in the US states a bit post-revolution (especially in Pennsylvania), but things generally remained quite conservative. Ratcliffe notes that age generally came into the discussion as part of the idea of 'manhood suffrage' which was a fairly radical idea that the right to vote was inherent within every adult male and shouldn't be qualified by land ownership.
Perhaps someone with more expertise could speak to this next example, but once we start getting back to the 17th century and earlier, we see things such as the French ancien regime having an age limit of 25 for the 'third estate' body (Crook). Albeit, as far as I am aware, the third estate was largely left out of real decision-making.
21 specifically then was inherited from British tradition in the United States and when it came up in political discussion re: voting rights was tied up in the idea of 'manhood suffrage'. How the British arrived at the age of 21 I personally am unable to speak on that with any expertise. But I hope what I've written answers your question to some extent.
I've also done the liberty of seeking earlier AH answers that might help. Answers giving more detail about why the age of adulthood is 18/21 in a general sense can be found here, and here, and here. They aren't US-specific but they might be informative in some way as to the background of adulthood age.
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