When did the U.S. President become "Mr. X" instead of "President X"?

Many media outlets in the USA refer to the President as "President X" on first reference and thereafter as "Mr. X".

National Public Radio has this policy:

Although many listeners find this second reference offensive, it is not a new policy. NPR has used "Mr." since the mid-1970s when President Gerald Ford was in office. The president is the only person whom NPR routinely refers to with the Mr. honorific on second reference. If NPR does a story, say on James Hamilton, an Ohio car dealer, he will be Hamilton on second reference, not Mr. Hamilton.

It appears this is the policy of the New York Times, as well: here's a September 18, 1851 New York Times article (PDF) that refers to President Fillmore as Mr. Fillmore on 2nd reference, and a July 6, 2011 New York Times article that does the same with President Obama.

The Corpus of Historical American English gives 62 hits for "Mr Roosevelt" between 1901 and 1910. A couple of those might be before he became president, and a few may concern other Roosevelts. But I am confident that the majority relate to President Roosevelt. There are 488 citations of "President Roosevelt" in the same period.

In the 1930's and 40's the corresponding figures are 7 and 5 for "Mr Roosevelt" and 1457, 1165 for "President Roosevelt".

This suggests to me that the trend you observe is actually returning to an older custom.

I can only confirm that this is US-specific. In France, nobody would ever say "Mr. Obama" (nor « Monsieur Obama »). Apparently, the UK calls him the same way as in France.

On a related note, while former French presidents were always called by their full name, (eg, « Jacques Chirac »), there was a change with the last President, which we call « Sarkozy », without his first name, even in the news.

Practice has varied greatly. Often, it's "President X" for the first mention, and "Mr. X" thereafter. Style guides sometimes disagree with each other.

It was often "General Washington" even when he was President, and afterwards - also true of Grant and Eisenhower.

It was Washington who requested "Mr. President" as the formal way of addressing him, specifically rejecting "Your Excellency" as contrary to the spirit of the Constitution. The practice continues. (the document itself is silent on forms of address)