Yes they are BUT I have seen them used for "at risk" adults not that this young lady has been classified as such. Just checked the Sherriff's, the Bay Net, & St. Mary's Today websites with no mention of her at all. Hope someone is not playing a cruel trick on us.
The reason Amber Alerts aren't used for adults, is because adults have the right to not report in, if they choose not to do so. I recently read an article, where a college student came home unexpectedly only to find her parents nowhere to be found. She reported them missing and the police checked into it. After they had been missing nearly a week, they police and news crews set up on the family's front lawn to hold a press conference. In the middle of the press conference, who should show up, but the parents. They had taken a spur of the moment trip to the mountains or some place, where they had no TV and had no idea they'd been reported missing.
One day, at a place where I worked, the police came looking for one of the employees. I wasn't there, but the man they were looking for was. They talked to him and left. Turns out, he'd been reported missing in New York 2 years before and he'd been tracked through his social security number. Apparently he'd gotten tired of his live in girlfriend, and was too big a coward to actually break up with her. He left most of his stuff, abandoned his job without notice, got in his car and drove away.
Adults are allowed to do that sort of thing. They don't have to report to anyone, so unless there is evidence of a crime, the person is not mentally capable or they leave behind important medications, the police aren't going to issue an alert. The whole point of an Amber Alert is they are extreme cases, otherwise the novelty would wear off and the general public will stop paying attention because people go missing everyday.
In fact, that the police issued this flyer by email DOES indicate they were concerned about this woman, probably because she'd been missing for 2 or 3 days. And apparently, they did the right thing because the woman is safe and sound.