I thought it would be because grown ups are now doing the jobs that were intended for teenagers.
those jobs weren't intended for teens, they just pay really low wages and that fit a teen's situation better than an adult because a teen normally doesn't have bills or dependents.
if the jobs were intended for teens, then the hours would reflect so. yet, plenty of minimum wage paying businesses are open 24/7, during school hours etc. if the jobs were meant for teens, then the business hours would be like maybe 4-6 hours in the afternoon only & longer hours on weekends.