lakewhut
> to be able to learn all day without interruptions
> is a risk that he falls into a bad crowd of other 20 year olds not learning.
both of these considerations have merit, but presumably these are for different people. Some may learn whole day (Steipler?) and some are at risk of falling into a bad crowd.
In our days, with so much affluence, the first one for sure should be given a chance to grow into a Talmid Chacham. Ad kan? One Rav I know was asked, when in yeshiva, to help a shul in his community once a week. Rosh Yeshiva told him to decline as “you will be more useful for them later on if you learn full time”.
For the second guy, you should think ahead and help him avoid problems now and also later, and that includes ability to earn an honest living and support a family, which implies either/or professional training and being able to work 8 honest hours. There is no reason this can not be done in safe environment.