The National Basketball Association (NBA) sometimes has players direct from high school (without playing college basketball), but it’s a professional league and players must grow up fast to have a career in it. The NBA nickname of “No Boys Allowed” has been cited in print since at least 1972, but became popularly used in the 1990s. Phil Jackson (who coached 11 NBA championship teams in Chicago and Los Angeles) is often credited for the saying, although he didn’t coin it.
 
   
Wikipedia: National Basketball Association
The National Basketball Association (NBA) is the pre-eminent men’s professional basketball league in North America, and is widely considered to be the premier men’s professional basketball league in the world. It has thirty franchised member clubs (29 in the United States and 1 in Canada), and is an active member of USA Basketball (USAB), which is recognized by FIBA (also known as the International Basketball Federation) as the national governing body for basketball in the United States. The NBA is one of the four major North American professional sports leagues. NBA players are the world’s best paid sportsmen, by average annual salary per player.
 
The league was founded in New York City on June 6, 1946, as the Basketball Association of America (BAA). The league adopted the name National Basketball Association on August 3, 1949, after absorbing the rival National Basketball League (NBL). The league’s several international as well as individual team offices are directed out of its head offices located in the Olympic Tower at 645 Fifth Avenue in New York City.
           
24 September 1972, Omaha (NE) World-Herald, “Here come the Kings” by Gary Johansen, Sunday World-Herald Magazine of the Midlands, pg. 24, col. 1:
Tom and Dick Van Arsdale are the best-known twin act in professional sports. They are stars in the National Basketball Association, a shoving, elbowing enterprise that taxes the heart and endurance of the mightiest of men. The NBA—No Boys Allowed.
 
5 March 1995, Orlando (FL) Sentinel, “No need to rush underclassmen” by Brian Schmitz, Sports, pg. C3:
You don’t bring an underdeveloped game, body or mind into the NBA, son. The NBA stands for No Boys Allowed.
 
7 December 1999, Salina (KS) Journal, “Hoop Heartache: Korleone Young hopes for second chance in NBA” by Van Williams (Wichita Eagle), pg. B2, col. 3:
As the saying goes, said Detroit Pistons assistant coach George Irvine, the NBA is also an acronym for “No Boys Allowed.”
 
‘We’re a league of men,” said Irvine, who worked with Young last season. “Nobody baby-sits you here. Korleone would have benefited a great deal if he had gone to college a year or two—not just in basketball, but in the maturation process.”
 
Sports Illustrated
May 21, 2001
Behind Closed Doors
IN ROUND 2 OF THE NBA PLAYOFFS, THE STAKES ARE HIGH, AND MOST TEAMS PLAY IT CLOSE TO THE VEST. BUT SI SHARED A WEEK WITH THE CHARLOTTE HORNETS AS THEY PLAYED THE BIGGEST GAMES OF THEIR BASKETBALL LIVES
L. Jon Wertheim
(...)
Twenty minutes before the tip-off Silas gathers the players in the locker room for a quick sermon. “Remember that NBA stands for No Boys Allowed,” he says. “Let’s have fun tonight. But you don’t have fun unless you’re kicking ass.”
 
Google Books
June 2001, Los Angeles (CA) magazine, pg. 144, col. 2:
Basketball, he (Los Angeles Lakers coach Phil Jackson.—ed.) noted, is a man’s game, and he recalled that, in his days with the Bulls, they’d taken to saying that the initials NBA stand for “No Boys Allowed.”
 
Twitter
I AM PEACE STAR
‏@StarburyMarbury
NBA.. No boys allowed. ORM only real men can speak on this channel. You only get a 146 words. Make it worth it.
2:55 PM - 21 Jul 09
   
ESPN
Phil Jackson takes shot at Heat
Updated: March 9, 2011, 1:09 PM ET
By Dave McMenamin | ESPNLosAngeles.com
ATLANTA—Los Angeles Lakers head coach Phil Jackson tried to bite his tongue.
(...)
The Heat were brought up again during Jackson’s pregame media session Tuesday night, however, and this time the Hall of Fame coach couldn’t help himself from tweaking the talented bunch from South Beach.
 
“This is the NBA: No Boys Allowed,” Jackson said. “Big boys don’t cry. But, if you’re going to do it, do it in the toilet where no one can see.”
 
This came seconds after Jackson tried to deflect the reporter’s question by saying, “People cry in locker rooms, yes, [but] I don’t want to talk about Miami’s situation.”
 
Daily News (New York, NY)
Former Knick Jerome Williams coaches Findlay Prep, one of country’s top high school basketball teams
Williams coaches an elite basketball program that has sent five players to the NBA in seven years, including Anthony Bennett, the top overall pick in this June’s draft. Three years ago, Tristan Thompson (fourth); Cory Joseph (29th) and DeAndre Liggins (53rd) were all selected by NBA teams and alumni of Findlay.
BY MITCH ABRAMSON / NEW YORK DAILY NEWS
PUBLISHED: SATURDAY, OCTOBER 5, 2013, 7:21 PM
UPDATED: SUNDAY, OCTOBER 6, 2013, 2:24 AM
(...)
Ron Naclerio, who’s coached four future NBA players at Cardozo high school in Queens, fears that players might be tempted to leave college before they are ready because of the constant references to the NBA they’re getting at the school.
 
“You know what the NBA stands for? No Boys Allowed,” Naclerio said. “When you’re still in high school you’re still considered a boy. They don’t realize. The NBA is a whole ’nother world.”
 
Delaware County Daily Times (Primos, PA)
Young: Sixers ‘nowhere near’ Brown’s vision
By Christopher A. Vito, Delaware County Daily Times
POSTED: 10/18/13, 9:16 PM EDT
(...)
“Charlotte’s not one of the better teams in the league and they beat us by 20-some points. It shows that this league is for real,” Young (Thaddeus Young of the Philadelphia 76ers.—ed.) said. “I remember hearing someone say ‘no boys allowed.’ That’s the title of the NBA. This is a grown-man’s game, and everyone has to grow up quick, fast and in a hurry if they want to be someone in this league.”