What gets me about "The Deadly Years" is the set of photos in The Making of Star Trek showing the age makeup process, with Shatner's "hair" being swept back and coated with makeup to create the very unconvincing illusion of it being thinner. He wouldn't even take off his toupee to simulate his character being bald. I wonder what the makeup team thought about that.
Kirk couldn't go bald in-universe, because he has to rush up to the bridge and save the ship after he is rejuvenated. I hardly think a bald captain could save a starship.