Prime Timeline
Leonard McCoy is the oldest human in Starfleet in 2364. He previously served as the chief medical officer of the original USS Enterprise and the USS Enterprise-A.
In 2364 he visited the USS Enterprise-D on its maiden voyage to Deneb IV, ferrying to and from that visit on board the USS Hood.
Viggo Timeline
Leonard "Old As Balls" McCoy is one of several Starfleet admirals that frequently makes use of the USS Hood in its capacity as the "admiral bus" to travel through Federation space.
2365
When Dr. Borik took a leave of absence from the Hood to deal with his pon-farr on Vulcan, McCoy was in the process of using the Hood as transportation. The admiral chose to assign himself as the acting chief medical officer of the Hood in Borik's absence, having felt fairly bored lately. He served in that role for roughly a year before Borik's return.