While reading Persian Mirrors* for school, I came across the fact of suffrage in Iran granted at 16 years of age. This fact is thematically tied into the ban on birth control and the call for more soldiers issued by Ayatollah Khomeini in the early 1980s. The call worked, and Iran's population went from a modest 35 million to 65 million, but the war with Iraq ended, and the religious authorities in Iran scaled back the prohibitions and controls, going so far as to approve vasectomies. Still, that's a solution for latter, and doesn't deal with the huge age imbalance in the nation. Iran's population is young, very, very young.
For comparison's sake, 1/4 of the Iranian population is below the age of 14, while only 1/5 of the United States population is. That doesn't appear to be a big disparity, but lets add that to the fact that the Iranian median age is 25, while in the US it's 36. This shows a disparity, but it isn't a disparity that easily translates into an explanation for Iran's voting age being 16, while the US (and most of the rest of the world) have suffrage set at 18. This is a cursory overview, as I think the full survey will work much better as late-college political science research.
But, the overview - going from the CIA world Factbook's list (which is pretty dang handy), the most popular option for voting is universal suffrage at 18, followed by universal suffrage at 18 with compulsory voting. The youngest voting ages are universal suffrage at 16 in Austria, Cuba and Iran, with suffrage for employed 16 year olds in Slovenia. Several nations have requirements of adulthood, without ages specified. Other nations have compulsory voting only for the married, or younger voting for the married. Still other nations have caps on compulsory voting at an old age, and make voting voluntary after that. We have a few nations (notably Japan) where suffrage is only granted at 19 or 20. And then, we have one interesting exception, found in many nations - soldiers (and sometimes even police) are forbidden from voting.
This list challenges my previous speculations about what entitles an individual to voting rights. Dying for one's nation seems to warrant the right of consent in government, but many nations specifically feel that those doing the dying shouldn't have the say, and so citizenship cannot be defined by potential for military service. Brazil has voting as compulsory for persons between 18 and 70, but optional for those 16-18 and over 70, which is an interesting twist on whose voice gets to be heard. Nonspecific adulthood is an interesting proposition, and it'd be interesting to see how a loose standard is applied in the daily exercise of politics. Some nations have education requirements, or education requirements for a specific gender (looking at you, Lebanon). And most interestingly, marriage as a requirement, which must be the result of the family, rather than the individual, seen as the essential component of society.
It's all very interesting, and at my present state I think that's all the interpretation I can offer. I'll hold that my previous criteria for voting as citizenship hold in the US, but it's pretty interesting to see how the question has been answered elsewhere.
* I can't really recommend the book, as it is too much a spoon-fed bipolar dichotomy aimed at separating Islamic government from Iran, and at explaining why the current Iranian government both isn't really Iranian and why it falls short of the universal standards every nation should abide by. By universal standards, I of course mean those of the enlightened west, and particularly the United States center-left. Not that the book doesn't have selling points (it was interesting enough to get me to write this tangentially related post), but it doesn't let other cultural standards be meaningful, and it treats a 1,300+ year old religious tradition in Iran as temporary and unessential to Iranian identity. Read Persepolis instead.