Always, not just since then. Since 1787, the President must be a natural born US citizen, but the criteria for being a natural born citizen goes back much farther. Even Vattel, in 1758, was only chronicling it, not establishing it.
What the law required in 1961, and which has since changed, is the requirement that in order for a child born outside the country to one citizen and one alien parent to be a citizen at birth (a naturalized citizen for Constitutional purposes), the US Citizen parent must have resided in the US for 10 years, 5 of them after their 14th birthday. BHO's mother was not yet 19 when he was born. Thus if born outside the US, he would not even be a citizen at birth, although he could have been naturalized later. The requirement is now 5 years total, 2 after the 14th birthday. But that does not matter. Even if both parents were US Citizens, if born overseas one is not a natural born citizen *unless* one or both parenst are overseas "in the armies of the nation" or it's diplomatic corps.