What am I doing wrong?

So this website, Texas State University - San Marcos (TxSt) - Reviews & Rankings, tells you the 4-year-graduation rates of different colleges. The graduation rates are listed at the bottom, and different colleges can be found by typing their name in the search-engine at the top. In the college that's linked, Texas State-San Marcos, it was only 24% of students graduate in 4 years. Now, on facebook, I've been looking up the ages of people who are upon graduation from these universities with low graduation rates. If the majority of these people take longer than 4 years, than the majority should be older than 22 upon graduating. However, this isn't what I've been finding. Almost everyone I've looked up who went to colleges with supposedly low graduation rates was 21-22 when graduating. What am I doing wrong? Where do I need to be looking to find results to accurately reflect these statistics?