The first recreational marijuana industry in the U.S. opened Wednesday in Colorado, kicking off an experiment that will be watched closely around the world.Activists hope to prove that legalization is a better alternative than the costly American-led war on drugs."Honestly, I thought I'd never see the day," said a giddy Errin Reaume, who shared hits of concentrated marijuana at a 1920s-themed "Prohibition Is Over" party in Denver.Skeptics worry the industry will make the drug more widely available to teens, even though legal sales are limited to adults over 21.Colorado voters in 2012 approved the legal pot industry. Washington state has its own version, which is scheduled to open in mid-2014. Uruguay passed a law in December to become the first nation to regulate pot.Pot advocates have argued a legal market would generate revenue for states and save money by not having to lock up so many drug offenders.At least 24 pot shops in eight Colorado towns opened, after increasing staff and inventory and hiring security."I'm going to frame the receipt when I go home, to remind myself of what might be possible. Legal everywhere," James Aaron Ramsey said, who served a brief jail sentence for pot possession less than a year ago and was excited to legally buy weed.A group of addiction counsellors and physicians said they're seeing more marijuana addiction problems, especially in youths, and that wider pot availability will exacerbate the problem."This is just throwing gas on the fire," said Ben Cort of the Colorado Center for Dependency, Addiction & Rehabilitation at the University of Colorado Hospital.Marijuana activists were hoping Colorado's experiment wouldn't be that noticeable after an initial rush of shopping."Adults have been buying marijuana around this country for years," said Mason Tvert, spokesman for the Marijuana Policy Project. "The only difference is that in Colorado they will now buy it from legitimate businesses instead of the underground market."Colorado Legalizes Recreational Pot