I would say it would have to be Q. ProfessorTrek 05:39, July 29, 2011 (UTC)
After some research, I found that the oldest individual whose birthdate was noted (not including individuals who exist in real life and are mentioned/seen in Star Trek) was Female Q, at 5 billion years old, as of 2373.
If you aren't counting immortals, then technically Flint, at 6,103 years old, as of 2269. (Flint was initially immortal, but because he left Earth and "all of its complex magnetic fields within which he had been formed" he lost his immortality, and died.)
The oldest mortal in Star Trek with a birthdate mentioned would be Wu, at age 462, as of 2268.
Gamma097 08:27, July 29, 2011 (UTC)