Ben Affleck Jets Off with J Lo & Kids to Celebrate 50th Birthday

Ben Affleck has a tendency to turn himself into hilarious memes -- and as he jets off with Jennifer Lopez and their kids to celebrate his birthday ... he seems to be reprising a classic.

The newlyweds boarded a private plane Saturday at the Van Nuys Airport in L.A. with some of their kids in tow, it seems. We recognize two of BA's children here, but it's unclear if Jen's young ones are already inside or even there at all. In any case, this trip is a family affair.

No word on where they'll ring in the big 5-0 for Ben -- his actual born day is on Monday -- but we're sure we'll find out soon enough. In the meantime ... what's with the long face???

Take a gander at Ben here ... who, in a majority of these shots, appears quite stern in his demeanor -- if not just outright bummed. Of course, him looking sorta sad here doesn't necessarily mean he's not excited for the festivities -- he probably is -- but it's funny to see this is how he looked just as he was getting ready to take in a huge milestone of life.

J Lo herself wasn't looking all that peppy either ... just matter-of-factly getting their crap from the car and making sure everyone/everything made its way up the steps on time.

Granted, these moments are the not-so-fun part of travel ... all the lugging and boarding and checking to see that all is in order before a major vacay, so that could be what's going on here. Still, though ... with Ben's history of appearing downtrodden, it's noteworthy.

There's the famous 'Batman v Superman' interview where he's stares off into space with a depressed gaze. Plus, his infamous bare-back beach stare-out ... and, of course, his eyes-closed cig moment from back then. The dude seems to channel gloom and doom a bit.