Megan Fox's Ex Brian Austin Green Tells MGK to Grow Up Amid Split

Brian Austin Green has a stern message for Machine Gun Kelly amid his recent split from Brian's ex-wife Megan Fox ... grow up!

We got Brian out in Calabasas on Thursday, and we asked him his thoughts on the now-former couple's most recent breakup. Brian claims to have not heard the news ... but after some thought, he questioned MGK's age -- implying maybe he's still very immature. Brian quipped to our camera, "Grow up! She's pregnant."

Brian says he wants the best for MGK and Megan's baby, which arrives in March 2025 ... adding he's heartbroken about the "tragic" situation because Megan and the kids they share together are really excited about the new baby's addition.

Brian said time will tell when we asked about the currently broken-up couple's future.

We broke the story ... Megan and MGK broke up during their Thanksgiving trip to Colorado after Megan discovered upsetting material on MGK's cell phone.