The D League age limit is 18 (unlike the NBA's age limit of 19), and there is no requirement that the player be out of high school at least one year (as there is in the NBA). The D League gets its players from a number of sources. Here are two, per its web site:
LOCAL TRYOUT PLAYERS
NBA D-League teams hold tryouts during each offseason, inviting some players to participate for free while attracting locals who pay a small fee to participate. The teams can invite the cream of the crop (up to five players) to their training camps.
A few dozen tryout players each year make their way onto training camp rosters, and many have even made it into game action. “Mr. Mad Ant” Ron Howard has authored the ultimate Cinderella story among that group, parlaying a training camp invitation in 2007 into a seven-year career in Fort Wayne, capped by
an historic championship season in 2014.
Examples: Kiwi Gardner (Santa Cruz Warriors), Ron Howard (Fort Wayne Mad Ants), Mario Little (Oklahoma City Blue)
NBA D-LEAGUE DRAFT PICKS
Every fall, more than 200 players are signed by the league office and placed in the NBA D-League Draft. About half of them are selected on Draft day, which features a field of NBA and NBA D-League vets, international pros, undrafted rookies and NBA D-League National Tryout players.
Many of the top picks arrive just days before the Draft after either arriving from overseas or being waived by NBA teams — a group that included Quincy Douby, James Johnson and DeAndre Liggins in 2013.
Examples (2013-14): Pierre Jackson (Idaho Stampede), Thanasis Antetokounmpo (Delaware 87ers), James Johnson (RGV Vipers)
Clearly, a high school player who felt he was good enough to play professionally but did not want to go to college could try out for the D League and, if he was good enough, could earn a spot to training camp or their way into the D League Draft. I repeat -- this would only work for a very small number of players. The vast majority of kids are going to be better served by playing college basketball, because they are not going to be good enough to be drafted by the NBA, or even good enough to get signed by the D League out of high school. But at least the D League opportunity is there as a backup. And the whole point of this discussion is that the "one-and-done" syndrome is caused by the NBA's age limit, which I would argue is an arbitrary and unnecessary restraint of a young man's opportunity to make a living.
PS--I would also argue the "one-and-done" syndrome is a minor problem compared to the mass transfers that occur every season. Fully 40 percent of college basketball players transfer before their junior seasons. That is far more disruptive to the game, in the big picture, than the handful of kids who play one season and then go to the NBA draft. I support the kids' rights to transfer, BTW. But there is no doubt the high transfer rate makes life chaotic for coaches and causes a lot of churn in programs.