I love 'Porridge'. I want to marry it and breed newts with it.
I agree with all here, it's timeless. Solid gold. Nay, platinum.
Again, great characters. Even the baddies being bizarrely likeable to the audience.
But I think the reason for it not ageing isn't down to not having any references to the era and being set in a prison.
I think the truly good stuff simply remains immune to changes in fashion and lifestyle.
Quality transcends.
One thing which also marks out 'Porridge' is the made up lingo. Nerks and scrotes and what-not
(one created a universe). Anything to get it past the BBC's censor bridgade.
Let's not forget, Mary Whitehouse must have been on the prowl those days.
As for 'stolen' gags, all sitcom constantly borrows from everywhere and everyone.
Nobody reinvents the wheel six times a season. Can't be done.
If we go back far enough half the stuff that's ever appeared in sitcom most likely originated in music hall variety acts decades earlier. (That said, I understand the point was about them stealing from themselves, effectively.)