I went through the tagged Post Apocalyptic stories and couldn't find this one plus I looked through some of the authors I thought it might be by. At any rate there is a couple of people who in their late 90s take part in a drug trial I think for curing Alzheimers and the drug works but in the case of two particular people they start to get younger and despite being initially wards, regain their freedom and decide to marry and live out a life together into what becomes an end of this world as you know it sort of deal. They move to a small farming town etc.
It was a well written story within say the last 4 or 5 years but my photographic memory ran out of film a few years ago so it slips by what it was called.
I do recall that part of it had a deal with descendants of George Washington serving the nation, that may spark a memory or three.