In australia we have a $12.50/hr minimum for 15 year olds. It climbs in increments until about $25/hr.
This system allows teenagers to get a foot in the door for low-skilled jobs, and allows adults to get paid a livable wage.
The result is that a lot of fast food jobs are performed by teenagers, and they are considered entry level jobs. Adults are then paid more, and take positions where they can add more value that what a teen can provide.