If you have a Class DJ junior license, and you complete a high school or college driver education course, you can change your Class DJ junior license to a Class D senior license when you are 17 years old.
Not before. Seems clear enough to me.
The rest of it:
To change the class of your driver license, you must have a driver education certificate (MV-285) issued from a DMV-approved driver education program. You cannot use another certificate.
If you do not have an MV-285, the DMV automatically changes your driver license from Class DJ to Class D on your 18th birthday. The DMV automatically mails you a new driver license that indicates "Class D". You will not get this document faster if you visit a DMV office.
On your 18th birthday, your driver record indicates that you are a Class D driver. There is no need to wait until you have your new Class D driver license before you drive as a senior driver. You can drive as a senior driver on your 18th birthday.
Give your driver education certificate (MV-285) to the DMV examiner after your road test. The DMV will update your driver record and will automatically issue a Class D photo driver license on your 17th birthday. You will receive the new driver license in the mail. There is no fee when you change the class of your driver license by this method.
Your driver license record will show that you are a Class D driver. You do not need to have your Class D document to drive as a senior license driver. You can drive on your 17th birthday.
Note: If your 17th birthday occurs on a weekend or a state holiday, your record is updated and your Class D driver license is issued on the next business day.