Pros
Security Guard jobs vary enormously. Allied, as the largest company, has a great variety of openings in many areas. Some assignments are horrible, some good for those who like to keep busy, some ideal for peace and quiet and almost complete absence of stress. Older people are ideal because their work ethic and basic experience in knowing what it means to do a good job and, above all, to show up on time, immediately put them in the top 75% of employees. Really! Many seniors are killing themselves working at Walmart or Jack in the Box when they could have reasonably satisfying, minimally demanding jobs in which their best qualities are *appreciated*. Go to work for a big contract security company like Allied, put up with being bounced around for six months, and you'll learn how to land an absolutely ideal slot. And if you are still motivated, advancement is ridiculously easy (although beware -- the stress of a supervisory position can be brutal). Pay is minimum wage.
Cons
There's no simple answer. Assignments can be absolutely horrible. Supervision and management likewise. Luck is involved, but the main thing is to stick with it while picking out what you want. With turnover as high as 50% a year, and many of your fellow workers lacking in every skill imaginable, your chance to become one of the most highly valued employees is excellent.