Aw, Mum and Dad, you're embarrassing me!
But it's done solely out of love.
They shared photo montages of their son as a baby. Victoria even included an ultrasound image of Brooklyn as a fetus.
"So on this day 18 years ago this little man came into our lives... To say we felt blessed is an understatement ... 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," David wrote, referring to an Irish boy band that enjoyed worldwide fame in the '90s.
"Thank fully he now sleeps the whole night without needing a song," he continued. "Brooklyn has gone from this beautiful baby boy to become this handsome , polite and driven young man .. 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 X ♥ @brooklynbeckham @victoriabeckham."
Brooklyn posted on his own page a photo of himself enjoying a beer. He is now allowed to drink alcohol legally in his native England.
"Pub time," he wrote.
"I can't believe our baby is 18 today," Victoria wrote. "We are all so proud and love u so much @brooklynbeckham x Happy Birthday Buster x Lots and lots of love x."
Brooklyn's 14-year-old brother Romeo Beckham, who recently joined Instagram, shared a photo of his big bro holding him when he was a baby.
"Brooklyn's birthday today and he has always taken care of me," he wrote. "♥love u brooklyn."