Taking to Instagram, the former England captain posted a montage of photos of Brooklyn, growing up from a little boy into the man he is today.
In another post, the father of four reflected on the huge role Brooklyn has played in his life over the past 18 years.
“From day one he never slept through the night, the only way he would fall asleep is when I took him out in the car and it had to be BoyZone on the radio and he would drop right off,” he wrote.
“Thankfully he now sleeps the whole night without needing a song.”
Meet the dad throwing a birthday party for all the kids who’ve never been invited to one. (Post continues after audio.)
“Brooklyn has gone from this beautiful baby boy to become this handsome, polite and driven young man,” he added.
“I’m proud that he lived through my career with me and I’m even prouder about the person that he has become.
“Happy birthday Bust, you can now officially take dad for a drink down the pub.”
Victoria Beckham has also wished her son a happy birthday on Instagram, saying “I can’t believe our baby is 18 today”.
Brooklyn is the big brother of Romeo James, Harper Seven and Cruz.