The council's initial approval reduces the penalties for the possession of an ounce of marijuana or less to only fines and no jail time for violators 18-21, the Denver Post reported. That would be a reduction from the current maximum punishment of up to a year in jail or hefty fines, the newspaper said.
Youths under 18 are sent to Denver's juvenile assessment center.
"This in no way is making it legal for these young people to possess or consume marijuana," Councilman Albus Brooks said. "What it does do is make it so that these kids don't have to live into adulthood with mistakes they might have made when they were 19."
Fines would jump from $150 for first-time infractions to $500 for a second offense and $999 for three or more offenses.
The City Council also gave preliminary approval to a measure that prohibits the display, consumption and growing of marijuana on city-owned property within 1,000 feet of a school.
Final votes on the measures are to be held next week.
In recent weeks, the City Council also approved allowing people in Denver to grow a dozen marijuana plants per household for recreational use and to smoke it on private property, even if visible from a public street.
The move to lessen the penalties surrounding the recreational use of marijuana comes ahead of the Jan. 1 rollout of the recreational marijuana industry.
Brooks said he pushed for the decriminalization measure because he saw inequity in how offenses are prosecuted.
"This evens the playing field," he said.
Although Assistant City Attorney David Broadwell said the occurrence rarely happens, those between 18 and 21 caught possessing less than an ounce can spend a year in jail or face fines up to $999.
Brooks said it is important for the city to have laws adjusted and enacted by the Jan. 1 deadline.
His bill, approved in committee last week, assigns the same penalties to those between 18 and 21 as are already in place for those over 21, the newspaper said.