US President Joe Biden signed gun-safety legislation into law that he called the most significant of its sort in decades, restricting firearm access for the youngest buyers and offering incentives for states to set up “red flag” laws.

“How many times have we heard that — ‘Just do something’?” Biden said in comments at the White House on Saturday. “Well, today, we did. While this bill doesn’t do everything I want, it does include actions I’ve long called for that are going to save lives.”

Congress approved the bill with unusual speed, two days after the Supreme Court struck down a New York gun law and ruled that the US constitution’s second amendment protects gun rights outside the home. Fifteen Republicans senators and 14 House Republicans joined all Democrats in voting to support it...