Completely self-serving point:
The human brain starts seriously reaching its final stages of development somewhere around the age of 25-30, depending on who you ask. So, being 24, I've started to notice that a lot of people around my age are in the final stages of their personalities becoming fully adult. I know one guy who's engaged to his fiance, and they basically live an "adult" life of trying to buy a house and working 8 hour per day jobs. They still do "childish" things, I guess, but it's the wrong way to think about it. You're basically seeing where they will end up.
You can always tell at this point who is seriously into pot, alcohol, or some other kind of drug because you can tell their brain is just not at the same stage as those who are becoming adults. The drug is stunting their mental development. The pot headz uzually talk like thisssss mannnnnn...... They look burnt out. The people who drink too much are impulsive and lack a meaningful emotional center.
You wish you could tell hard drinkers and pot heads in high school "Dude, if you don't stop this, you literally won't be able to become an adult." But then you would just be the man, trying to stomp out their good times.
An employer screening for THC use is basically saying "who among these job applicants is a child masquerading as a man?"