NRA spokeswoman Dana Loesch said the gun lobby does not believe the age for purchases should be raised from 18.
Federal law includes several different age limits for purchases. Licensed gun dealers cannot sell a handgun to a person younger than 21, but can sell a rifle to anyone 18 or older. Unlicensed sellers, such as those at gun shows, can sell handguns to those 18 or older and rifles regardless of age.
"It should all be at 21," Trump said Thursday, predicting that the NRA would support that limit. He has repeated that call several times.
Loesch, on ABC's "This Week," tried to minimize any policy differences with the White House, saying: "I know that people are trying to find daylight between President Trump and 5 million law-abiding gun owners."
The man authorities identified as having shot and killed 17 people, mostly teenagers, in a high school in Parkland, Fla., is 19.
The debate after that attack has the NRA on the defensive as a boycott campaign widens, aimed at businesses with ties to the group.
Politicians, too, have been called out by a movement spearheaded by student activists, including some survivors of the attack at Marjory Stoneman Douglas High School. A few elected officials whose positions have long aligned with those of the NRA, including Trump and Florida Gov. Rick Scott, have expressed newfound support for raising the age limit for gun sales.
Many congressional Republicans have remained silent on the issue, however. Some have supported Trump's suggestion that some teachers should be issued weapons and trained in their use, an idea that has drawn strong criticism from educators.
"What you need is some of the teachers who are armed," Rep. Thomas Massie, R-Ky., said on NBC's "Meet the Press" Sunday.
Massie also voiced strong opposition to weapons-free zones in and around schools, and likened calls for more stringent background checks to putting "lipstick on a pig."