Normally, they would not wait that long to file charges unless there was something unique about your case.
There's a chance that the paperwork got lost in the shuffle somewhere along the line.
Of course, it could be that the case is being taken very seriously and they're still investigating, checking the victim's condition, waiting for lab analysis work (DNA) to be completed, etc.
I wouldn't suggest you contact the DA directly - it may remind them to look for your paperwork and not miss the statute of limitations. If you're really curious, you can consult with a criminal defense attorney in your area - there are ways we can check without arousing suspicion.
DO NOT CONTACT THE D.A.-NO SENSE IN WAKING THEM UP. YOUR CASE IS EITHER LOST IN THE SHUFFLE OR BEING WORKED UP. GBI CASES, EVEN WITH A BB GUN, USUALLY DO NOT LIE AROUND. ANY NUMBER OF GOOD ATTORNEYS IN SACTO CAN QUIETLY ASCERTAIN YOUR CASE'S STATUS, ESPECIALLY IF YOU ARE VENUED IN ROSEVILLE.