You don't have to "get" a new license when you turn 21. You'll renew your driver's license on or before its expiration date, which in your case will be your birthday in August 2021. DL's are usually issued for 6 or 8 years, and they expire 6 or 8 years later, on the holder's birthday.
If you're referring to obtaining a "real ID" DL: Beginning on October 2020, all new and renewed licenses will be "real ID's". Since you already have a valid license, you don't have to do anything - it will automatically renew in August 2021 as a "real ID". Until you renew, you present license will still be valid as a DL and personal ID, but it won't be valid for Federal ID purposes.
If you want, you can take your present license to the DMV and exchange it for a "real ID" now. Everything on that license will remain the same - the license no. and the expiration date. The only difference will be that your new "real ID" will have a star in the upper right hand corner.
Some states began issuing all new or renewed licenses as "real ID's" 3 or more years ago. Take a look at your present license. If it has a star on it, you already have a "real ID".