No, not in criminal court, but the DMV will use it against this person. There are a couple of ways of potentially preventing the DMV from using it. If the person is currently under 21, s/he can petition the court to dismiss the prior under welfare and institutions code section 782. That can be followed by a petition to have the entire juvenile record sealed and destroyed. Either or both of these may prevent the DMV from finding out about the juvenile case.
No, usually the prior juvenile conviction cannot be used against him/her in adult court. One problem persists in this situation... The prosecutor can always look into someone's past record. Whether it's legal or not does not matter. You will never know unless the prosecutor "publicly publishes" the prior juvenile record without a court order. This means that the prosecutor comments on the prior juvenile record in open court. If this happens, report that prosecutor to the bar immediately.
My office is relentless in keeping out any comments or reports that dive into my client's past juvenile record in criminal court.
As far as the DMV, this might or might not count as a prior DUI for licensing matters. I would need more information to fully evaluate the status of the DMV case.