Good question actually! In my opinion, it can be as scary as YOU make it or as fun as YOU make it. It just depends on what you like and how you react to the different elements of fear Universal throws at you. For example I've seen plenty of 20-40 year olds literally trampling each other in fear to get out of the haunted mazes, while I've seen 10 and 12 year olds having a blast.
The first year you attend is always the scariest IMO because you don't know what to expect. Every year though, there is sure to be plenty of jumps and "boo!" moments! Universal also does an excellent job of creating a safe, party atmosphere. Security is usually great and suprisingly even though alchohol is served there is usually very, very few problems with fighting, rowdiness, etc.
I've found that most of the GOOD scares actually come in the streets. In the mazes you KNOW people will be jumping out at you all the time. In the streets you don't really know what to expect and tend to let your gaurd down a bit.
Have fun,
Jim