Since I'm a crusty middle-aged ****, I had no problem whatsoever with the bigger names in this film being middle-aged or even a bit older than that.
You've seen Guy Standeven in something because the man was in everything.

Case in point: compare John Carpenter's "The Fog" with its remake.

That's because movies back then we're meant for adults to enjoy too. Many current mainstream movies are geared for younger audiences.

They're over fifty and STILL ALIVE??

working behind the scenes
That's the rub. To paraphrase Hamlet.

Charlton Heston made several action/adventure movies in the seventies when he was in his fifties. Among them were "Planet of the Apes," "Earthquake," and "The Omega Man." Lorne Greene was nearly sixty, though he was playing the father of Mr. Heston's wife who was only seven years his junior. George Kennedy (still alive at 90) had just turned fifty.
If you look around you today you will see many people in responsible positions who are in their fifties. Many of us work in the organization that will come to save you in the event of a natural or man-made catastrophe. While most of us who are in our fifties or sixties will be working behind the scenes managing the response, some of the fire department battalion commanders, police precinct captains, lead EMTs, field grade national guard officers, and volunteers are in their fifties.