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

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.

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.