Lindsay Lohan soberly celebrated her 21st bday with fam, friends and once and former boytoy -- but photogs were not allowed to get near the fun -- forced instead to stake out the sand while Lindz blew out her candles.

Paparazzi started assembling seaside at the house around 2:00 PM and were present until the last presents were unwrapped -- around 2:15 AM. A paparazzo's work is never done.