Hull life depends on use as well as age

Submarines make the stakes of hull life especially clear because their pressure hulls have a finite history of dives and need intensive inspection before service can be extended. Ships within one class do not necessarily age alike because diving depths and other operating demands may have imposed different stresses. Early units can also embody design mistakes corrected as the builders gain experience, allowing a later ship from an older class to outlast the first ships of its successor class. A replacement decision therefore needs a ship-specific view of structural and design history, not merely a class-wide age figure. Even a feasible structural extension still needs upgradeability, mission value, and crewing benefits large enough to justify its cost.