The hinge on my 5 month old MacBook Pro is very loose. I took it to the Apple store, where I was told that it will tighten over time. Has anyone else had the same problem? Has the hinge tightened over time? This sounds suspect, due to the fact he said the spring inside gets tighter, but I was taught in school that springs lose their tension over time.
No, it will not tighten overtime. If anything, it can only become looser as it wears (simple physics).
You should book an appointment with a Genius at the Apple Store and tell them you are unhappy with it. They will either replace the mechanism or give you a new machine.
The previous person you dealt with, unfortunately gave you false information.