Could also be that alcohol is like 2x as expensive over the past 30 years.
b) we're talking youth - who are nowadays also less likely to be manning the fast food counters - I remember working at 14 years old and now in many states that's not permitted until 16. So even less disposable income.
If teens and young adults NEEDED to work but couldn't find jobs, the unemployment rate for that age group would show it.