DENVER — Serenity Christensen, 14, is too young to set foot in one of Colorado’s many marijuana shops, but she was able to spot a business opportunity in legal weed. She is a Girl Scout, and this year, she and her mother decided to sell their cookies outside a dispensary. “Good business,” Serenity said.

But on the other side of Denver, legalization has turned another high school student, David Perez, against the warehouselike marijuana cultivations now clustered around his neighborhood. He said their skunky aroma often smacks him in the face when he walks out his front door.

These are the ripples of five years of legal marijuana. Colorado’s first-in-the-nation experiment has reshaped health, politics, rural culture and criminal justice in surprising ways that often defy both the worst warnings of critics and blue-sky rhetoric of the marijuana industry, giving a glimpse of what the future may hold as more and more states adopt and debate full legalization.

Since recreational sales began in 2014, more people here are visiting emergency rooms for marijuana-related problems, and hospitals report higher rates of mental-health cases tied to marijuana. At the same time, thousands of others make uneventful stops at dispensaries every day, like the hiking guide in the college town of Boulder who now keeps a few marijuana gummies in a locked bag to help her relax before bed.