An 18 year old can qualify for medicaid especially if a parent signs the 18 year old up for medicaid and the parents income level is low enough to qualify for medicaid.
Or if the teen has a disability bad enough to put the teen in a nursing facility.
If you're 18 and living on your own then it will depend on the state that you live in on whether you'll qualify for medicaid or not.
If you can afford private health insurance then private health insurance would be a much better option than medicaid.