If you want to get into bowhunting, the most important thing about the bow you use is that it fits you. There are ALL KINDS of things that enter into whether or not a bow "fits" a person, you can look around on the forum and see the many threads dealing with this same topic. As far as old or new, the only real consideration there would be whether or not there is a lot of wear on the bow, and also make sure there is someone around who can work on it if need be (accessability of replacement parts should enter into this as well).
I hunted with a PSE I got when I was 14 up until I was in my 20's... Then I used my dad's bow that was at least 10 years old for a few seasons. I finally bought a "new" bow that was a 2005 model. I know one guy here on the forum has used his same bow for like 15 years or something... bottom line: You need to find a bow that you like, is comfortable to draw/hold/shoot, and that you can be accurate with. I'd suggest you shoot the various bows available, then choose the one that most matches your criteria for comfort and price.
"When a hunter is in a tree stand with high moral values and with the proper hunting ethics and richer for the experience, that hunter is 20 feet closer to God." ~Fred Bear