Star Trek: The Next Generation (1987)

S1E15 Too Short a Season

Stardate: 41309.5. Starfleet Admiral Mark Jameson, who is in the twilight of his life and battling a serious illness, is in the midst of discussions to secure the freedom of hostages. Unexpectedly, he experiences a mysterious transformation that causes him to regain his youth, altering the course of his negotiations in unforeseen ways.