There is a special forum further down the main page where you can find all sorts of interesting threads on the topic of daycare.
In answer to your question, I really think this is an extremely personal decision. For me, I held out until DS was over 12mo - even though both DH and I could see that psychologically I would personally be better off back at work.
If we were ever lucky enough to have number 2, I would probably try and get as close as possible to 12mo again. It made me happy to think he could crawl and pull himself up to see what was on the activity tables, and that for me, outweighed the desire to return to work.
But I have many colleagues who have cheerfully put their children in daycare from 3 or 4 months old, and have no regrets about doing so.
Of course you want your little one to be happy, but part of their happiness is "whole family happiness" - which they may not be getting if you are constantly stressed about your studies.
Also, if they were genuinely too young...the centre would say so. All centres have a minimum age (usually either 6 weeks or 3 months), and that is really the minimum at which the centre says before that they are too young and the centre cannot adequately cater to their needs. Once you have passed your centre's minimum age then it is really a very personal choice of what is best for your whole family.