I am licensed in PA and do not recall any questions about past drug use on the bar application. I quickly looked at the application under character and fitness on the Illinois bar site, and I do not see any questions about past drug use there either.
I do see questions about whether you have committed a crime, including low level crimes involving drugs, or high level traffic offenses. Lying about such a thing would be quite serious, and yes, the applicant could be disbarred for it.
Lying on a bar application is very serious. While bar examiners often forgive mistakes made in youth, they will expect to see signs of improved behavior and learning from past mistakes by the time you take the bar. Lying would be a sign that such lessons had not been learned.
Whether the person would be disbarred I do not know. But they could easily be prevented from taking the exam in the first place, or more likely, if it is caught during the character and fitness review, which often occurs after the bar is successfully completed, the person would be refused admission to the bar or expected to explain himself.