There are several kinds of people attracted to this work:
1.) The "Lifer": A loser in life who cannot do anything other than be a waste of space. These are the ones that make it harder for other people to perform a simple job of making sure a client's property doesn't burn down or get ransacked. They are insecure, lazy, and mentally slow. They think that they are somehow like Police or the Military. These tend to be in their late 30's and higher but you can see this sad excuse for existence in some 20 somethings that really fail at life. .
2.) College students: A lot of the younger guards you will run into who seem intelligent are usually in this class. Often this occupation allows for ample study time if done covertly. The pay rate is much better than most college jobs so they play it smart and do these kinds of jobs.
3.) Retired people: Surprisingly enough not every old guard you see will be an idiot. Often these are guys and gals who have retired and in order to have a supplemental income, they take on this job. They are often articulate and fairly intelligent for the most part.