Instagram / @tombrady

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

See Full Page