Absorption fails first, and the rent gap is a symptom of the same thing.
Start with the pool. 4,100 people 65-plus is not 4,100 prospects. Convert to households, roughly 2,600 to 2,900 in a market with that ownership rate, then screen for who can pay $1,450. At a 35% shelter burden, $1,450 needs about $50,000 of income, which is well above your $38,000 median. Senior renters cover part of that from assets rather than income, and that's the piece your market can't supply generously: a $145,000 house nets maybe $130,000 after costs, which funds the gap for a while and then doesn't. Run the qualified count both ways, income-only and income-plus-home-proceeds, and see how many households survive.
Then note that your qualified pool is a one-time stock. Absorption of 8 a month for six months means you capture nearly everyone who was ready to move, and after that you're living off new qualifiers each year, which in a county of 31,000 is a small annual number. Small-market age-restricted lease-up more commonly runs 3 to 5 a month, so plan 10 to 14 months of carry and size the interest reserve for it. That's usually the line that breaks these deals rather than stabilized NOI.
Build it in phases if the site allows. Twenty units, prove the rent, hold the rest of the pad.
The risk you haven't named is who else has this idea. In rural markets the credible second entrant is often the hospital foundation or a church-affiliated nonprofit, and they can build at a cost of capital you cannot match and price under you indefinitely. Ask around locally about anyone doing a feasibility study before you option the ground.
