I don’t know why I read this into the OP- since the OP himself in no way specifies it- but I was thinking “younger actors” meaning are there actors “of today” who have combat experience specifically in comparison to the significant number of professional actors who served in Word War II. For that generation, there was a great number of well known actors with combat experience: Jimmy Stewart, Henry Fonda, Gene Autry, Ernest Borgnine, Burgess Meredith, Jack Palance, the list goes on.
So, that’s the frame of reference I was working with as I considered the OP’s question. Again, the OP did not actually specify as such. I don’t know why I read into it that way. So, seeing someone list Schwarzenegger didn’t strike me as out of place.
Likewise, as per the OP: “I wonder, how many and which of the current set of actors have served in combat?” So long as THE EXPENDABLES 2 is currently on-screen, sure as the big guy seems to have two other movies currently in post-production and two more currently in pre-production, I figured it was worth mentioning.