Its very well thought out, each marker is placed in the year the person is born and depending on which way the bench is facing indicates if they died on the ground or in the plane. The youngest person to die was a three year old little girl, along with her parents. Their marker is together and its the very first one you come upon. The oldest person to die was 7, his was the last bench in the memorial.
At the beginning there was a sign with a telephone number you could call and listen to a pre-recorded introduction about the memorial I listed to the shorter version that was 12 minutes and got some really awesome insight to the memorial.
Other close by places worth the visit in my opinion: Arlington Cemetery and the Air Force memorial.
Here are a few pictures I took along the way. (P to the S: the DO NOT LIKE when you take pictures of the Pentagon!!)
Looking across the different benches
This is the memorial to the youngest death and her parents
Each person's year of birth was a granite marker along the wall
Then each bench was placed in the year that they were born. You can sort of see the silver line indicating that is a year.