I don't have a definitive memory of this by any means, and searching old websites in 2018 is like pulling teeth now that Google "unlaunched" custom date ranges (yet another step on the long journey toward making Google search the pinnacle of vapid "relevance"). However, I seem to remember first seeing it on left-leaning political blogs in the early '00s as mockery of the pattern of complaining about "the gays", "the Jews", "the blacks", etc.. So when pundits complained about welfare, it was paraphrased as ranting about "the poors", when they railed against pensions or Medicare they were angry with "the olds", and so on. It's a very short hop from there to people posting comments that start like "As a poor, [...]".

Somebody's trying to fit in and say what they heard somebody else say. Thence are these things born and propagated.

This one takes an adjective and makes it a noun. Sounds more like 1950s lingo (a black, a white, a gay etc.) but with the illusion of freshness with its being applied to old people for the first time. Call me a rude, but it's a sucky. At least "old person" acknowledges they're a person.

I think it's more a simulation of an older person using awkward youth slang.

At least, when I see it, it's usually an older person saying something like "As an old I really don't get this fashion for tattoos that look like someone scribbled random doodles all over you."

It's not supposed to be; it is.

I've even seen older people pick it up. It's a good syntax.

I mostly see older people use it in online contexts that are perceived as predominantly younger. "As an old ..."