I just saw an article where an employer said he'd no longer hire entry-level workers, they're just too expensive.
That's the issue in a nutshell.
Employers pay workers what they are worth, not what workers 'need', ideally. The incompetent (entry-level) worker has the same needs (rent, food, car, etc) as the expert worker, so should they both make the same wage? Of course not.
If it becomes illegal to pay a worker less than $18/hr, then jobs that aren't worth $18/hr will go away, and where will high school graduates with no discernible skills (yet, entry-level workers) learn those skills if they cost the employer just as much as a competent (experienced) worker?
Requiring a fast-food restaurant to pay a clerk $25/hr to stand at the counter to punch orders into a POS terminal will have the effect of eliminating that job, to be replaced by a kiosk. You may or may not prefer kiosks, but that counter worker was learning skills and good work habits, now where do they go?