It's hard to tell how old she is just by looking at her. At 16 inches she could be a large juvenile/subadult or she could be a full grown adult dragon (my 2 yr old female is only 16 inches).
Regardless of how old she is, you'll want to give her a large daily salad that's mostly dark leafy greens (collard greens, dandelion greens, endive, escarole, turnip greens, mustard greens, etc.) with veggies and a bit of fruit mixed in for variety. There's a helpful food list available at
http://www.beautifuldragons.503xtreme.com/Nutrition.html
The area where age makes a difference is in live food. A young dragon eats much more live food than an adult dragon does. For instance, with a growing dragon her size you could safely give 10-20 superworms a day, while an adult should only have 25-30 supers a week.
You could try feeding here the increased amount of food as if she were a juvenile for a month or two and keep a close eye on her weight and length. If she doesn't get any longer then it's pretty safe to say that she's an adult who is done growing. If she is a juvenile after all, the extra food will allow her to continue growing in length.
