Once you turn 18 the Sacramento DA generally does not file in juvenile court. Once you turn 18, the DA will usually files in adult court. Juvenile court is for minors, although a juvenile can be subject to juvenile court supervision until age 25. If the crime occurred when the individual was under age 18, I'm not sure what the DA's office would do.
If a juvenile committed a crime while he was under 18 and, lets say, a warrant went out for failing to appear. Three plus years pass and he is over 21 today, he can still be arrested on the case and brought before a juvenile judge. The court still has jurisdiction over him; however, usually these cases are negotiated and settled because they cannot offer any services to him and they would not want to offer him services. If he had a signficant past and this was a 707 case where he could be filed on as an adult, the DA would do so. These cases rarely occur, but I have seen it happen now and then.
For a complete answer to a Criminal based questions you should consult a lawyer with all the facts concerning what occurred. No promises or guarantees can be made with any legal case. This answer is meant only as informational, not legal advice.