Dear Boyd,  

It does seem a real Catch-22, you can't enter until you are 21, but if you go to college your loans will not allow you to enter when you are old enough... 

First, look into all the different provinces of the group you are interested in, there may be a variety of policies and some province might accept you before college.  

Generally religious orders that have minimum age restrictions such as you mention are aware of the type of problem you face, and have tried to work out some solution. Some have programs that you can enroll in that are almost like a candidacy - they tell you where you can go to college so as to live with the community on or off campus, by living with them you can grow in their spirit and also keep your debts to a minimum and you avoid the other distractions of college. 

Some groups will tell you what to study in college, and if you follow their advice and are accepted by them when you are old enough, they take care of the loans after you get ordained.  

You have to inquire, and do your homework.