Job Fairs come in various formats and reflect the state of the market. Some companies actually look for staff, others simply gather CVs. Be selective and use a targeted approach.
Do some preliminary research on the firms, try to find out how well they are doing. Find out if they have signed any new contracts recently or have they been merged or acquired. Find out if they have laid people off recently, etc. This will let you know how to set your expectations. Look at who is staffing the booth.
Twenty-year-olds are often junior HR types that are using the job fair to practice - or as a marketing exercise! If there are managers or operational types in attendance, there may be openings within the firm and that you will be able to pitch yourself for a job. Here is a place where you can use your benefits statement.
Be open to leads, do not discount ANY opportunity. Wear a suit and a smile, - check your body language, leave your attitude @home!
Remember that a job fair is an opportunity to network not only with the companies, but with other job seekers as well. The benefits may not be immediate. One of our group found a job at a job-fair after having met the same recruiter for her third time.