An age restricted Multi-Family Complex, is merely an apartment complex with a minimum age requirement, typically 55 years old, and older.
This is treated as a simple apartment complex, and financed like one. Like an ILF, rents can be subsidized by HUD, or fully borne by the tenant. Often times, entire buildings are subject to HUD / HAP contracts vis-à-vis the rents, in the case of affordable housing.
Like any real estate asset, qualities, and location, are the dominant factors in determining value, and consequently financing.