Text for Old Posts

Hi, apologies if this has been covered elsewhere, but I’ve not found the answer by searching.

My blog has been running for 12 years now and I have lots of old posts, many of which link to external pages and sites which no longer exist. I’d like to put a disclaimer text on posts more than a year old which warns readers that I don’t monitor all external links and I can’t promise that they will work. There is no obvious up-to-date plugin that will do this, but I suspect that it could be coded into the template, if only I was better with php. Does anyone have an ideas of the best way forward?

Many thanks

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