Tom Brady Wishes Son Jack Happy 18th Birthday, 'We Love You So Much'

Significant milestone for Tom Brady's family ... Jack, his oldest kid, turns the big 1-8 today -- and the NFL legend celebrated with a heartfelt post on social media, welcoming his boy to adulthood!

Brady shared the Insta post Friday morning, accompanied by a carousel of photos showing Jack through the years, from when he was a young buck until now.

"From little steps to big dreams, happy birthday Jack," Brady wrote. "You've always shown grace, courage, love and kindness since God blessed us with your arrival. I'm so proud of the man you are!"

Brady had to squeeze some dad humor in there, too ... joking he'll get to deal with all the fun that comes with officially being an adult.

"Welcome to adulthood, it comes with higher credit card limits, bigger bills, and no curfew… OK maybe not all at once," he said.

The 48-year-old wrapped up his post telling his son how loved he is, and saying this will be the "best year yet" for Jack.

"There is a saying that a dad's heart soars with his children's happiness, not with his own…truer words could not be spoken."

And, Brady seemed to learn his lesson from earlier in the week when he posted a photo of Jack towering over him ... prompting some ex-teammates to clown him. In the newly posted set, dad's taller.