WASHINGTON—Lamenting his first teenage cigarette, President Barack Obama ruefully admitted on Monday that he’s spent his adult life fighting the habit. Then he signed the nation’s toughest anti-smoking law, aiming to keep thousands of other teens from getting hooked.

Obama praised the historic legislation, which gives the Food and Drug Administration unprecedented authority to regulate what goes into tobacco products, to make public the ingredients and to prohibit marketing campaigns geared toward children.

But he didn’t say how his own struggle was coming since he moved into the White House. And aides were no more forthcoming.

As senator, candidate and now president, Obama has veered between frank and cagey about his personal battle with smoking.

He promised his wife, Michelle, more than two years ago that he would quit if she let him seek the White House.