When a rebuild effectively creates a new ship

Does rebuilding a warship reset its age?

Only an almost complete structural rebuild would do so in practical terms, and that is usually comparable to building a new ship.

The answer distinguishes an ordinary refit from reconstruction of the hull framing and most of the ship's structure. Replacing equipment or sections does not make the underlying hull young again. Rebuilding the entire frame could effectively reset much of the vessel's physical age, even if its official identity remained unchanged. In practice, work on that scale approaches the cost and effort of constructing a new ship, which is why navies seldom use it merely to extend service life.