It’s a bit cliche to say you’re never too old to learn. Fact is, it may take a bit longer to overcome the beliefs and habits that he’s been building in his 70+ younger years and build new ones. But if he takes appropriate action while going through the program, the subliminals work as well for him as they do for everybody else. The human mind is flexible, you just need to wake it up and direct it.
He should experiment a bit with the masked/ultrasonics to see what works better for him though. His ears have been around for 70+ years as well.
EDIT: I’d actually be curious to see what Limitless could do for him in this case. Combined with Mogul, maybe?