"We appreciate the NBA's decision to provide additional opportunities for those who would like to pursue their dream of playing professionally", NCAA president Mark Emmert said in a statement.
While changes to the NBA's age limit likely won't be implemented until the 2022 draft, the league considers this an immediate response.
Players will be eligible to sign the select deal if they turn 18 by September 15 prior to the season that they would spend in the G League.
The vast majority of prospects enrol in a college programme, while some play overseas for a year and return to the United States to be drafted.
Turner said the move addresses that concern.
The G League has allowed 18-year-old players in the past, but never before under any elite designation.
Earlier this year, Los Angeles Lakers star LeBron James called the NCAA model "corrupt" and said he would suggest to NBA Commissioner Adam Silver a plan to expand the G League and turn it into more of a farm system with an eye on truly preparing young talent for the NBA. Some prospects - perhaps several - still will desire the college basketball pit stop, where they can receive top-tier coaching, immeasurable marketing opportunities (mostly via so many of their games being on national television and, of course, the NCAA Tournament) and a bridge between their high school lives and basketball as a full-time job.
WNBA players can opt out of their current collective bargaining agreement with the league by October 31.
The NBA G League, the organisation's second-tier league, will offer $US125,000 ($174,850) "select contracts" for a five-month season to elite players who are at least 18 years old. It will take some time for a baseline to be established for what prospects can be expected to produce against this level of competition. It will be fascinating to see how the college basketball world reacts to competition for its biggest stars and most marketable athletes, especially as the sport is engulfed in an ongoing corruption trial. Givony reports that the league will "target recent or would-be high school graduates who otherwise would have likely spent just one season playing college basketball".