Brad's fans started the festivities by hopping online to fawn over him -- just one scroll online lands you in a sea of comments dubbing him the "sexiest man alive" and "my forever crush" ... with others demanding to know his secret for staying young.

It's supposed to be an extra special milestone for Brad ... who'll most likely be blowing out the candles alongside his much younger girlfriend of one year, Ines de Ramon.

Jewelry designer Ines is 27 years his junior -- and this is his first proper relationship since his public divorce from Angelina Jolie.

Sources also told People last month they're in a good place ... with the actor introducing Ines as his girlfriend to others -- that she makes him very happy.