So I've been instructed to write an essay that has citations for research that must be within the last 5 years. The exact phrasing is "Minimum of 20-30 up to date (within 5 years) references". One of the key and major concepts was defined in the 70s and simply can't be avoided in the discussion. Is it cheating to cite a paper from recent times, which discusses another research article from that older time?
The function of this essay (as I gather) is for you to get familiar with recent literature and show your instructor that you read & understood it.
Minimum of 20-30 up to date (within 5 years) references
It does not mean only use refs from last 5 years. It means "don't copy-paste from textbooks many of which references papers that were published 10+ years ago".
In science there are usually couple of very old foundational papers, few important ones, but only the most recent present modern view on concepts and experiments. So it makes more sense to read few old papers, and many more very recent ones. It makes no sense to read the mediocre papers from 15 years ago, better read mediocre papers from 1-5 years ago.
Did the instructor say all references must be from the last five years, or just some?
If it is the former, then that is not a sensible instruction, but I would cite the recent paper and also say "this is a recent description but actually the concept was first defined in the 1970s".
If it is the latter, cite the 1970s paper, and also the recent one if you want.