Are high protein drinks like muscle milk bad for a very healthy and fit old man (74)?

They could be very bad for his kidneys. At 74 you do not need to add more work for the kidneys to do . See a nutritionist.

If the gentleman is very healthy and fit at 74 years old, don't change his diet at all.

My parents use to eat a lot of cakes, pies, cupcakes, candy, and I thought that wasn't a very good diet, but then it drawn on me, they were already in their mid-to-late 90's, so apparently it was working for them.