According to this authoritative source, Bill Shatner started wearing an on-screen toupee in 1957 as his hairline began to recede while he was still in his twenties. No one has been able to find a photo of Shatner sans toupee taken later than 1959.
Obviously, Kirk and Sulu were retrieving photographic evidence besides that from Captain Christopher's wing cameras.
Haha. It always makes me think of kids I went to grade school with, but at least in that case, you could blame their mothers. There's just no good reason for a grown man to have that haircut, IMO..