There is no requirement to replace old wiring unless new changes are being done to an old circuit that does not meet current code, in this case all the old wiring in that circuit must be replaced.
It is entirely up to you if you want to replace old wiring or leave it as is (even if it looks unsafe) when no new changes are being made to the old circuit (e.g. you open a wall to do plumbing work and find antique "knob and tube" wiring with insulation problems - you do NOT need to replace any of this old wiring if you did not change its circuit in any way).
Many houses in this area have unsafe 2 prong outlets installed in the 1950s that were designed for use interchangeably on 115V 15A and 230V 8A circuits, but their design allows 115V 15A, 115V 20A, or 230V 8A plugs to be inserted without the user knowing they are plugging into an incorrect circuit. But unless something else is changed on a circuit containing these unsafe outlets they may all be left as they are, but every one of these unsafe outlets on the circuit must be replaced with safe 2 prong if anything is changed (3 prong if the circuit is upgraded to 3 wire grounded wiring).
About the only thing that could force rewiring the entire building is replacing an old screw in fuse box with a modern breaker box, as this changed to feed end for all circuits.
----------------------------------------------------------------------------------------------------------------------------------------------If the wiring has rubber insulation for the conductors you should definitely replace it with modern pvc-based wiring. To protect your electrical outlets and appliances - and also for safety reasons - you should also upgrade the breaker box. If you have old wires which are made of copper that has pvc-based insulation you can leave them as they are but only if they are still in good condition. If the wires are made of aluminum or if they don't look safe - for instance if the insulation is damaged - you should replace them immediately.
Old wiring may be safe if left alone, not overloaded or abused, and was originally "up to code". However, old rubber insulation with cloth coverings can become brittle and literally turn to dust if jiggled. This often occurs in overhead lighting outlets where heat from lights has dried and damaged the insulation. These old installations must be replaced if they are to be touched in any serious way (e.g., undoing clamps or other fasteners), otherwise they become unsafe. In other words, you should not simply pull out the old fixtures and devices and install new ones; you will need new wiring, and if you're getting new wiring, you should (must) get new circuit breakers.
You can expect your main electrical panel and meter to last 30 to 40 years before they start to need to be replaced. The wiring in the wall can last a bit longer but when it gets to be 50 to 60 years old you're better off replacing it if you can.
In the late 1940s a type of cable called "Romex" was introduced in the United States, using PVC insulation for the individual copper conductors with a paper wrap and braided fabric overwrap. It is usually black or silver in color. This type of wiring is still safe today, although early Romex cables didn't contain a third earth ground wire, as required by current electrical codes. Houses with this type of wiring usually have outlets with just two slots, lacking the third hole for 3-prong plugs, although sometimes homeowners have replaced the outlets with the 3-prong type in violation of the National Electric Code (NEC). Current Romex cables have a third copper conductor that connects to earth ground and the outer jacket is molded PVC plastic instead of braided fabric.
Take a look at your fuse box or breaker box. If it still contains the old screw-in fuses, chances are that the wiring in the house is outdated and should all be upgraded to modern Romex or "NM" type cable.
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