oh it is definitely suitable for a 13 year old! Of course with a guardian...it gets extremely crowded at the parks. In addition, most rides are open during the event...so riding Jaws, or seeing T2:3-D at midnight could be appealing as well.
The price is $35 per person for one night of the event, or $58 per person for both. If you had an Annual Pass, you would receive a discount on the tickets. They have other price options if you need to buy a pass for the daytime as well.
Universal's RTU event site than tell who would be playing, and a little about the group, in addition to more information about the prices.
www.rocktheuniverse.com
Some of the top bands playing are TobyMac, ZoeGirl, Relient K, Kutless, and Newsboys...all ranging in different styles.

Keep in mind these are Christian music artists. If you are interested, the 13-year old would be surrounded by many well-behaved and spiritual kids, teens, and even adults. I attended it last year, and found it to be a great and safe environment. Disney's Night of Joy was a little more chaotic IMHO.