They were old, some came online in the 60s. Check the situation in France right now -- half of their aging fleet is offline because they basically can't get them to work. The NY Times had an article on it recently.
Last 10 gigawatts of shut down reactors were all finished in 80's and probably had half of their design age left, all the really old were shut down already in nineties.
But the industry has tumbled into an unprecedented power crisis as EDF confronts troubles ranging from the mysterious emergence of stress corrosion inside nuclear plants to a hotter climate that is making it harder to cool the aging reactors.
The outages at EDF, Europe’s biggest electricity exporter, have sent France’s nuclear power output tumbling to its lowest level in nearly 30 years, pushing French electric bills to record highs just as the war in Ukraine is stoking broader inflation.
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The article also mentioned EDF might need to be nationalized since it's in so much debt, and in fact this was announced a few days ago.
No, you can't maintain them forever. The EDF is mismanaged, in Germany we never had this kind of problems with nuclear plants. South Korea also has an excellent track record with building new plants.