According to Wikipedia:
From around 1950, single sentence spacing became standard in books, magazines and newspapers. However some sources still state that additional spacing is correct or acceptable. The debate continues, notably on the World Wide Web—as many people use search engines to try to find what is correct. Many people prefer double sentence spacing for informal use because that was how they were taught to type. There is a debate on which convention is more readable, but the few recent direct studies conducted since 2002 have produced inconclusive results.

I have to say, I use a double space after a period, and I'm only 34, so it is not (as Googled tried to tell me) proof that you are over 40 if you use a double space!

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