Justin Bieber -- Celebrating 21st Early With A Sweet '65

Justin Bieber is getting his 21st b-day started by partying with a 50-year-old ... his buddies surprised him with a sick '65 Lincoln Continental.

Sources close to the situation tell TMZ ... Biebs was rollin' up and down the Sunset Strip Thursday night in style after his boys John and Sammy Shahidi surprised him at the studio with the completely restored classic.

We're told the Shahidi's spent several months working on the whip -- valued around $20K -- and looks just like the car in "Entourage" ... one of Bieb's favorite shows.

The gift bar's been set pretty high ... but you still have time before March 1st to take back that desktop calendar.

We reached out to the owners of "Shots" for comment -- so far no word back.