This Rules Roundtable covers it pretty well. The main bullet points are:

• We do not allow an individual Redditor's personal experience or personal anecdotes as answers here. The more recent events become, the greater the chance of that happening is.

• Modern politics are ... contentious, and we try to stay away from them as much as possible.

• Historical remove is a concern of ours. It's often said that journalists write the first draft of history, and while that is true, the obvious corollary to that is that later drafts of history have a better chance of understanding the context and importance of events than "on the ground" stories do.

Now the obvious exception to that is our rule on historiography (the study of history), where it's absolutely appropriate to talk about trends in the present day, sources in the present day, etc.

For some suggestions about getting questions < 20 years ago answered, I'll just plagiarize from the other thread:

For political questions, /r/Ask_Politics and /r/NeutralPolitics are a good place to look, as is /r/GeoPolitics or /r/IRStudies.

For questions regarding people and society, /r/AskSocialScience covers a number of disciplines, including Political Science, Public Policy, and Sociology.

For current events, /r/TrueReddit doesn't allow text submissions, but does attempt to maintain intelligent discussion on the articles present, which can make for good kick offs. They is also the subreddit /r/InsightfulQuestions, which shares a similar mission, and mod team with TR. /r/TrueAskReddit also it worth mentioning there. While not strictly an "Ask" style subreddit, /r/ChangeMyView provides a forum for civil discourse on weighty matters. /r/CredibleDefense focuses on military affairs and conflicts, generally. Additionally, there are a number of subreddits which devote themselves to certain events, such as /r/SyrianCivilWar or /r/JihadInFocus.

The twenty-year convention is grounded in the goal of making the study of history as objective as possible, recognizing that when people study people a certain amount of subjectivity will be creep into the discussion. The general assumption is that journalists and political scientists cover the present to the near past, and then when it begins to be possible to look at things a bit more objectively, the topic is handed off to historians. Again, recognizing that the line is blurred and that journalists and political scientists frequently reach beyond the twenty-year barrier, and historians frequently comment on more recent events.

The US National Park Service exercises a fifty-year convention when evaluating the significance of resources for listing in the National Register of Historic Places and/or as National Historic Landmarks. The rationale behind this convention, as established in 1967, was that people could not properly evaluate the significance of a resource until sufficient time had elapsed. There is an exceptions rule since some places and resources are so clearly significant that they can be evaluated before the passage of five decades, but listing of resources before they turn fifty requires an added level of justification and administrative review.

There is a subtle difference separating the two barriers: the twenty-year rule is a discipline-wide convention intended to promote objectivity; the fifty-year rule is intended to acknowledge that while we may be able to view the past more objectively, the significance of events and resources may not come into sharp enough focus until half a century has passed. In other words, we can now - in theory - begin to talk about the Clinton impeachment with level of objectivity, but it takes a bit longer to make certain that a resource of only twenty years is timeless enough to justify including it in a semi-permeant register of things that are important to the nation.